forked from Minki/linux
d0807da78e
Immediate flag has been used to disable per-task consistency and patch
all tasks immediately. It could be useful if the patch doesn't change any
function or data semantics.
However, it causes problems on its own. The consistency problem is
currently broken with respect to immediate patches.
func a
patches 1i
2i
3
When the patch 3 is applied, only 2i function is checked (by stack
checking facility). There might be a task sleeping in 1i though. Such
task is migrated to 3, because we do not check 1i in
klp_check_stack_func() at all.
Coming atomic replace feature would be easier to implement and more
reliable without immediate.
Thus, remove immediate feature completely and save us from the problems.
Note that force feature has the similar problem. However it is
considered as a last resort. If used, administrator should not apply any
new live patches and should plan for reboot into an updated kernel.
The architectures would now need to provide HAVE_RELIABLE_STACKTRACE to
fully support livepatch.
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
492 lines
21 KiB
Plaintext
492 lines
21 KiB
Plaintext
=========
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Livepatch
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=========
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This document outlines basic information about kernel livepatching.
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Table of Contents:
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1. Motivation
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2. Kprobes, Ftrace, Livepatching
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3. Consistency model
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4. Livepatch module
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4.1. New functions
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4.2. Metadata
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4.3. Livepatch module handling
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5. Livepatch life-cycle
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5.1. Registration
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5.2. Enabling
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5.3. Disabling
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5.4. Unregistration
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6. Sysfs
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7. Limitations
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1. Motivation
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=============
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There are many situations where users are reluctant to reboot a system. It may
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be because their system is performing complex scientific computations or under
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heavy load during peak usage. In addition to keeping systems up and running,
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users want to also have a stable and secure system. Livepatching gives users
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both by allowing for function calls to be redirected; thus, fixing critical
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functions without a system reboot.
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2. Kprobes, Ftrace, Livepatching
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================================
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There are multiple mechanisms in the Linux kernel that are directly related
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to redirection of code execution; namely: kernel probes, function tracing,
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and livepatching:
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+ The kernel probes are the most generic. The code can be redirected by
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putting a breakpoint instruction instead of any instruction.
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+ The function tracer calls the code from a predefined location that is
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close to the function entry point. This location is generated by the
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compiler using the '-pg' gcc option.
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+ Livepatching typically needs to redirect the code at the very beginning
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of the function entry before the function parameters or the stack
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are in any way modified.
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All three approaches need to modify the existing code at runtime. Therefore
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they need to be aware of each other and not step over each other's toes.
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Most of these problems are solved by using the dynamic ftrace framework as
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a base. A Kprobe is registered as a ftrace handler when the function entry
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is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
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a live patch is called with the help of a custom ftrace handler. But there are
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some limitations, see below.
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3. Consistency model
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====================
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Functions are there for a reason. They take some input parameters, get or
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release locks, read, process, and even write some data in a defined way,
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have return values. In other words, each function has a defined semantic.
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Many fixes do not change the semantic of the modified functions. For
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example, they add a NULL pointer or a boundary check, fix a race by adding
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a missing memory barrier, or add some locking around a critical section.
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Most of these changes are self contained and the function presents itself
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the same way to the rest of the system. In this case, the functions might
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be updated independently one by one.
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But there are more complex fixes. For example, a patch might change
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ordering of locking in multiple functions at the same time. Or a patch
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might exchange meaning of some temporary structures and update
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all the relevant functions. In this case, the affected unit
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(thread, whole kernel) need to start using all new versions of
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the functions at the same time. Also the switch must happen only
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when it is safe to do so, e.g. when the affected locks are released
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or no data are stored in the modified structures at the moment.
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The theory about how to apply functions a safe way is rather complex.
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The aim is to define a so-called consistency model. It attempts to define
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conditions when the new implementation could be used so that the system
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stays consistent.
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Livepatch has a consistency model which is a hybrid of kGraft and
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kpatch: it uses kGraft's per-task consistency and syscall barrier
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switching combined with kpatch's stack trace switching. There are also
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a number of fallback options which make it quite flexible.
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Patches are applied on a per-task basis, when the task is deemed safe to
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switch over. When a patch is enabled, livepatch enters into a
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transition state where tasks are converging to the patched state.
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Usually this transition state can complete in a few seconds. The same
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sequence occurs when a patch is disabled, except the tasks converge from
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the patched state to the unpatched state.
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An interrupt handler inherits the patched state of the task it
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interrupts. The same is true for forked tasks: the child inherits the
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patched state of the parent.
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Livepatch uses several complementary approaches to determine when it's
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safe to patch tasks:
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1. The first and most effective approach is stack checking of sleeping
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tasks. If no affected functions are on the stack of a given task,
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the task is patched. In most cases this will patch most or all of
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the tasks on the first try. Otherwise it'll keep trying
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periodically. This option is only available if the architecture has
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reliable stacks (HAVE_RELIABLE_STACKTRACE).
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2. The second approach, if needed, is kernel exit switching. A
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task is switched when it returns to user space from a system call, a
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user space IRQ, or a signal. It's useful in the following cases:
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a) Patching I/O-bound user tasks which are sleeping on an affected
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function. In this case you have to send SIGSTOP and SIGCONT to
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force it to exit the kernel and be patched.
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b) Patching CPU-bound user tasks. If the task is highly CPU-bound
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then it will get patched the next time it gets interrupted by an
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IRQ.
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3. For idle "swapper" tasks, since they don't ever exit the kernel, they
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instead have a klp_update_patch_state() call in the idle loop which
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allows them to be patched before the CPU enters the idle state.
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(Note there's not yet such an approach for kthreads.)
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Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
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the second approach. It's highly likely that some tasks may still be
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running with an old version of the function, until that function
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returns. In this case you would have to signal the tasks. This
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especially applies to kthreads. They may not be woken up and would need
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to be forced. See below for more information.
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Unless we can come up with another way to patch kthreads, architectures
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without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
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the kernel livepatching.
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The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
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is in transition. Only a single patch (the topmost patch on the stack)
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can be in transition at a given time. A patch can remain in transition
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indefinitely, if any of the tasks are stuck in the initial patch state.
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A transition can be reversed and effectively canceled by writing the
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opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
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the transition is in progress. Then all the tasks will attempt to
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converge back to the original patch state.
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There's also a /proc/<pid>/patch_state file which can be used to
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determine which tasks are blocking completion of a patching operation.
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If a patch is in transition, this file shows 0 to indicate the task is
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unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
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transition, it shows -1. Any tasks which are blocking the transition
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can be signaled with SIGSTOP and SIGCONT to force them to change their
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patched state. This may be harmful to the system though.
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/sys/kernel/livepatch/<patch>/signal attribute provides a better alternative.
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Writing 1 to the attribute sends a fake signal to all remaining blocking
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tasks. No proper signal is actually delivered (there is no data in signal
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pending structures). Tasks are interrupted or woken up, and forced to change
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their patched state.
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Administrator can also affect a transition through
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/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
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TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
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state. Important note! The force attribute is intended for cases when the
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transition gets stuck for a long time because of a blocking task. Administrator
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is expected to collect all necessary data (namely stack traces of such blocking
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tasks) and request a clearance from a patch distributor to force the transition.
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Unauthorized usage may cause harm to the system. It depends on the nature of the
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patch, which functions are (un)patched, and which functions the blocking tasks
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are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
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modules is permanently disabled when the force feature is used. It cannot be
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guaranteed there is no task sleeping in such module. It implies unbounded
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reference count if a patch module is disabled and enabled in a loop.
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Moreover, the usage of force may also affect future applications of live
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patches and cause even more harm to the system. Administrator should first
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consider to simply cancel a transition (see above). If force is used, reboot
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should be planned and no more live patches applied.
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3.1 Adding consistency model support to new architectures
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---------------------------------------------------------
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For adding consistency model support to new architectures, there are a
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few options:
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1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
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for non-DWARF unwinders, also making sure there's a way for the stack
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tracing code to detect interrupts on the stack.
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2) Alternatively, ensure that every kthread has a call to
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klp_update_patch_state() in a safe location. Kthreads are typically
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in an infinite loop which does some action repeatedly. The safe
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location to switch the kthread's patch state would be at a designated
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point in the loop where there are no locks taken and all data
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structures are in a well-defined state.
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The location is clear when using workqueues or the kthread worker
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API. These kthreads process independent actions in a generic loop.
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It's much more complicated with kthreads which have a custom loop.
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There the safe location must be carefully selected on a case-by-case
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basis.
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In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
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able to use the non-stack-checking parts of the consistency model:
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a) patching user tasks when they cross the kernel/user space
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boundary; and
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b) patching kthreads and idle tasks at their designated patch points.
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This option isn't as good as option 1 because it requires signaling
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user tasks and waking kthreads to patch them. But it could still be
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a good backup option for those architectures which don't have
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reliable stack traces yet.
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4. Livepatch module
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===================
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Livepatches are distributed using kernel modules, see
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samples/livepatch/livepatch-sample.c.
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The module includes a new implementation of functions that we want
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to replace. In addition, it defines some structures describing the
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relation between the original and the new implementation. Then there
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is code that makes the kernel start using the new code when the livepatch
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module is loaded. Also there is code that cleans up before the
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livepatch module is removed. All this is explained in more details in
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the next sections.
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4.1. New functions
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------------------
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New versions of functions are typically just copied from the original
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sources. A good practice is to add a prefix to the names so that they
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can be distinguished from the original ones, e.g. in a backtrace. Also
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they can be declared as static because they are not called directly
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and do not need the global visibility.
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The patch contains only functions that are really modified. But they
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might want to access functions or data from the original source file
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that may only be locally accessible. This can be solved by a special
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relocation section in the generated livepatch module, see
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Documentation/livepatch/module-elf-format.txt for more details.
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4.2. Metadata
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-------------
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The patch is described by several structures that split the information
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into three levels:
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+ struct klp_func is defined for each patched function. It describes
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the relation between the original and the new implementation of a
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particular function.
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The structure includes the name, as a string, of the original function.
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The function address is found via kallsyms at runtime.
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Then it includes the address of the new function. It is defined
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directly by assigning the function pointer. Note that the new
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function is typically defined in the same source file.
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As an optional parameter, the symbol position in the kallsyms database can
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be used to disambiguate functions of the same name. This is not the
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absolute position in the database, but rather the order it has been found
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only for a particular object ( vmlinux or a kernel module ). Note that
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kallsyms allows for searching symbols according to the object name.
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+ struct klp_object defines an array of patched functions (struct
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klp_func) in the same object. Where the object is either vmlinux
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(NULL) or a module name.
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The structure helps to group and handle functions for each object
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together. Note that patched modules might be loaded later than
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the patch itself and the relevant functions might be patched
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only when they are available.
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+ struct klp_patch defines an array of patched objects (struct
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klp_object).
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This structure handles all patched functions consistently and eventually,
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synchronously. The whole patch is applied only when all patched
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symbols are found. The only exception are symbols from objects
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(kernel modules) that have not been loaded yet.
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For more details on how the patch is applied on a per-task basis,
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see the "Consistency model" section.
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4.3. Livepatch module handling
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------------------------------
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The usual behavior is that the new functions will get used when
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the livepatch module is loaded. For this, the module init() function
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has to register the patch (struct klp_patch) and enable it. See the
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section "Livepatch life-cycle" below for more details about these
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two operations.
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Module removal is only safe when there are no users of the underlying
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functions. This is the reason why the force feature permanently disables
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the removal. The forced tasks entered the functions but we cannot say
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that they returned back. Therefore it cannot be decided when the
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livepatch module can be safely removed. When the system is successfully
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transitioned to a new patch state (patched/unpatched) without being
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forced it is guaranteed that no task sleeps or runs in the old code.
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5. Livepatch life-cycle
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=======================
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Livepatching defines four basic operations that define the life cycle of each
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live patch: registration, enabling, disabling and unregistration. There are
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several reasons why it is done this way.
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First, the patch is applied only when all patched symbols for already
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loaded objects are found. The error handling is much easier if this
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check is done before particular functions get redirected.
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Second, it might take some time until the entire system is migrated with
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the hybrid consistency model being used. The patch revert might block
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the livepatch module removal for too long. Therefore it is useful to
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revert the patch using a separate operation that might be called
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explicitly. But it does not make sense to remove all information until
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the livepatch module is really removed.
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5.1. Registration
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-----------------
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Each patch first has to be registered using klp_register_patch(). This makes
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the patch known to the livepatch framework. Also it does some preliminary
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computing and checks.
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In particular, the patch is added into the list of known patches. The
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addresses of the patched functions are found according to their names.
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The special relocations, mentioned in the section "New functions", are
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applied. The relevant entries are created under
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/sys/kernel/livepatch/<name>. The patch is rejected when any operation
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fails.
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5.2. Enabling
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-------------
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Registered patches might be enabled either by calling klp_enable_patch() or
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by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
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start using the new implementation of the patched functions at this stage.
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When a patch is enabled, livepatch enters into a transition state where
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tasks are converging to the patched state. This is indicated by a value
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of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have
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been patched, the 'transition' value changes to '0'. For more
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information about this process, see the "Consistency model" section.
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If an original function is patched for the first time, a function
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specific struct klp_ops is created and an universal ftrace handler is
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registered.
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Functions might be patched multiple times. The ftrace handler is registered
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only once for the given function. Further patches just add an entry to the
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list (see field `func_stack`) of the struct klp_ops. The last added
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entry is chosen by the ftrace handler and becomes the active function
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replacement.
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Note that the patches might be enabled in a different order than they were
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registered.
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5.3. Disabling
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--------------
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Enabled patches might get disabled either by calling klp_disable_patch() or
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by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
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either the code from the previously enabled patch or even the original
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code gets used.
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When a patch is disabled, livepatch enters into a transition state where
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tasks are converging to the unpatched state. This is indicated by a
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value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks
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have been unpatched, the 'transition' value changes to '0'. For more
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information about this process, see the "Consistency model" section.
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Here all the functions (struct klp_func) associated with the to-be-disabled
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patch are removed from the corresponding struct klp_ops. The ftrace handler
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is unregistered and the struct klp_ops is freed when the func_stack list
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becomes empty.
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Patches must be disabled in exactly the reverse order in which they were
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enabled. It makes the problem and the implementation much easier.
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5.4. Unregistration
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-------------------
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Disabled patches might be unregistered by calling klp_unregister_patch().
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This can be done only when the patch is disabled and the code is no longer
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used. It must be called before the livepatch module gets unloaded.
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At this stage, all the relevant sys-fs entries are removed and the patch
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is removed from the list of known patches.
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6. Sysfs
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========
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Information about the registered patches can be found under
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/sys/kernel/livepatch. The patches could be enabled and disabled
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by writing there.
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/sys/kernel/livepatch/<patch>/signal and /sys/kernel/livepatch/<patch>/force
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attributes allow administrator to affect a patching operation.
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See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
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7. Limitations
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==============
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The current Livepatch implementation has several limitations:
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+ The patch must not change the semantic of the patched functions.
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The current implementation guarantees only that either the old
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or the new function is called. The functions are patched one
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by one. It means that the patch must _not_ change the semantic
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of the function.
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+ Data structures can not be patched.
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There is no support to version data structures or anyhow migrate
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one structure into another. Also the simple consistency model does
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not allow to switch more functions atomically.
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Once there is more complex consistency mode, it will be possible to
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use some workarounds. For example, it will be possible to use a hole
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for a new member because the data structure is aligned. Or it will
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be possible to use an existing member for something else.
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There are no plans to add more generic support for modified structures
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at the moment.
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+ Only functions that can be traced could be patched.
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Livepatch is based on the dynamic ftrace. In particular, functions
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implementing ftrace or the livepatch ftrace handler could not be
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patched. Otherwise, the code would end up in an infinite loop. A
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potential mistake is prevented by marking the problematic functions
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by "notrace".
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+ Livepatch works reliably only when the dynamic ftrace is located at
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the very beginning of the function.
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The function need to be redirected before the stack or the function
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parameters are modified in any way. For example, livepatch requires
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using -fentry gcc compiler option on x86_64.
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One exception is the PPC port. It uses relative addressing and TOC.
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Each function has to handle TOC and save LR before it could call
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the ftrace handler. This operation has to be reverted on return.
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Fortunately, the generic ftrace code has the same problem and all
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this is handled on the ftrace level.
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+ Kretprobes using the ftrace framework conflict with the patched
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functions.
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Both kretprobes and livepatches use a ftrace handler that modifies
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the return address. The first user wins. Either the probe or the patch
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is rejected when the handler is already in use by the other.
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+ Kprobes in the original function are ignored when the code is
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redirected to the new implementation.
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There is a work in progress to add warnings about this situation.
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