forked from Minki/linux
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We have observed that on very large machines with newer CPUs, the static key/branch switching delay is on the order of milliseconds. This is due to the required broadcast IPIs, which simply does not scale well to hundreds of CPUs (cores). If done too frequently, this can adversely affect tail latencies of various workloads. One workaround is to increase the sample interval to several seconds, while decreasing sampled allocation coverage, but the problem still exists and could still increase tail latencies. As already noted in the Kconfig help text, there are trade-offs: at lower sample intervals the dynamic branch results in better performance; however, at very large sample intervals, the static keys mode can result in better performance -- careful benchmarking is recommended. Our initial benchmarking showed that with large enough sample intervals and workloads stressing the allocator, the static keys mode was slightly better. Evaluating and observing the possible system-wide side-effects of the static-key-switching induced broadcast IPIs, however, was a blind spot (in particular on large machines with 100s of cores). Therefore, a major downside of the static keys mode is, unfortunately, that it is hard to predict performance on new system architectures and topologies, but also making conclusions about performance of new workloads based on a limited set of benchmarks. Most distributions will simply select the defaults, while targeting a large variety of different workloads and system architectures. As such, the better default is CONFIG_KFENCE_STATIC_KEYS=n, and re-enabling it is only recommended after careful evaluation. For reference, on x86-64 the condition in kfence_alloc() generates exactly 2 instructions in the kmem_cache_alloc() fast-path: | ... | cmpl $0x0,0x1a8021c(%rip) # ffffffff82d560d0 <kfence_allocation_gate> | je ffffffff812d6003 <kmem_cache_alloc+0x243> | ... which, given kfence_allocation_gate is infrequently modified, should be well predicted by most CPUs. Link: https://lkml.kernel.org/r/20211019102524.2807208-2-elver@google.com Signed-off-by: Marco Elver <elver@google.com> Cc: Alexander Potapenko <glider@google.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Jann Horn <jannh@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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.. SPDX-License-Identifier: GPL-2.0
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.. Copyright (C) 2020, Google LLC.
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Kernel Electric-Fence (KFENCE)
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==============================
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Kernel Electric-Fence (KFENCE) is a low-overhead sampling-based memory safety
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error detector. KFENCE detects heap out-of-bounds access, use-after-free, and
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invalid-free errors.
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KFENCE is designed to be enabled in production kernels, and has near zero
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performance overhead. Compared to KASAN, KFENCE trades performance for
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precision. The main motivation behind KFENCE's design, is that with enough
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total uptime KFENCE will detect bugs in code paths not typically exercised by
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non-production test workloads. One way to quickly achieve a large enough total
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uptime is when the tool is deployed across a large fleet of machines.
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Usage
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-----
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To enable KFENCE, configure the kernel with::
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CONFIG_KFENCE=y
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To build a kernel with KFENCE support, but disabled by default (to enable, set
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``kfence.sample_interval`` to non-zero value), configure the kernel with::
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CONFIG_KFENCE=y
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CONFIG_KFENCE_SAMPLE_INTERVAL=0
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KFENCE provides several other configuration options to customize behaviour (see
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the respective help text in ``lib/Kconfig.kfence`` for more info).
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Tuning performance
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~~~~~~~~~~~~~~~~~~
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The most important parameter is KFENCE's sample interval, which can be set via
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the kernel boot parameter ``kfence.sample_interval`` in milliseconds. The
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sample interval determines the frequency with which heap allocations will be
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guarded by KFENCE. The default is configurable via the Kconfig option
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``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``kfence.sample_interval=0``
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disables KFENCE.
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The KFENCE memory pool is of fixed size, and if the pool is exhausted, no
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further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default
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255), the number of available guarded objects can be controlled. Each object
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requires 2 pages, one for the object itself and the other one used as a guard
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page; object pages are interleaved with guard pages, and every object page is
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therefore surrounded by two guard pages.
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The total memory dedicated to the KFENCE memory pool can be computed as::
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( #objects + 1 ) * 2 * PAGE_SIZE
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Using the default config, and assuming a page size of 4 KiB, results in
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dedicating 2 MiB to the KFENCE memory pool.
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Note: On architectures that support huge pages, KFENCE will ensure that the
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pool is using pages of size ``PAGE_SIZE``. This will result in additional page
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tables being allocated.
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Error reports
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~~~~~~~~~~~~~
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A typical out-of-bounds access looks like this::
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==================================================================
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BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa6/0x234
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Out-of-bounds read at 0xffff8c3f2e291fff (1B left of kfence-#72):
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test_out_of_bounds_read+0xa6/0x234
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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kfence-#72: 0xffff8c3f2e292000-0xffff8c3f2e29201f, size=32, cache=kmalloc-32
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allocated by task 484 on cpu 0 at 32.919330s:
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test_alloc+0xfe/0x738
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test_out_of_bounds_read+0x9b/0x234
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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CPU: 0 PID: 484 Comm: kunit_try_catch Not tainted 5.13.0-rc3+ #7
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
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==================================================================
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The header of the report provides a short summary of the function involved in
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the access. It is followed by more detailed information about the access and
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its origin. Note that, real kernel addresses are only shown when using the
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kernel command line option ``no_hash_pointers``.
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Use-after-free accesses are reported as::
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==================================================================
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BUG: KFENCE: use-after-free read in test_use_after_free_read+0xb3/0x143
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Use-after-free read at 0xffff8c3f2e2a0000 (in kfence-#79):
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test_use_after_free_read+0xb3/0x143
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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kfence-#79: 0xffff8c3f2e2a0000-0xffff8c3f2e2a001f, size=32, cache=kmalloc-32
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allocated by task 488 on cpu 2 at 33.871326s:
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test_alloc+0xfe/0x738
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test_use_after_free_read+0x76/0x143
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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freed by task 488 on cpu 2 at 33.871358s:
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test_use_after_free_read+0xa8/0x143
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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CPU: 2 PID: 488 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
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==================================================================
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KFENCE also reports on invalid frees, such as double-frees::
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==================================================================
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BUG: KFENCE: invalid free in test_double_free+0xdc/0x171
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Invalid free of 0xffff8c3f2e2a4000 (in kfence-#81):
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test_double_free+0xdc/0x171
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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kfence-#81: 0xffff8c3f2e2a4000-0xffff8c3f2e2a401f, size=32, cache=kmalloc-32
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allocated by task 490 on cpu 1 at 34.175321s:
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test_alloc+0xfe/0x738
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test_double_free+0x76/0x171
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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freed by task 490 on cpu 1 at 34.175348s:
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test_double_free+0xa8/0x171
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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CPU: 1 PID: 490 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
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==================================================================
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KFENCE also uses pattern-based redzones on the other side of an object's guard
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page, to detect out-of-bounds writes on the unprotected side of the object.
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These are reported on frees::
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==================================================================
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BUG: KFENCE: memory corruption in test_kmalloc_aligned_oob_write+0xef/0x184
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Corrupted memory at 0xffff8c3f2e33aff9 [ 0xac . . . . . . ] (in kfence-#156):
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test_kmalloc_aligned_oob_write+0xef/0x184
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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kfence-#156: 0xffff8c3f2e33afb0-0xffff8c3f2e33aff8, size=73, cache=kmalloc-96
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allocated by task 502 on cpu 7 at 42.159302s:
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test_alloc+0xfe/0x738
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test_kmalloc_aligned_oob_write+0x57/0x184
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kunit_try_run_case+0x61/0xa0
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x176/0x1b0
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ret_from_fork+0x22/0x30
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CPU: 7 PID: 502 Comm: kunit_try_catch Tainted: G B 5.13.0-rc3+ #7
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
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==================================================================
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For such errors, the address where the corruption occurred as well as the
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invalidly written bytes (offset from the address) are shown; in this
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representation, '.' denote untouched bytes. In the example above ``0xac`` is
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the value written to the invalid address at offset 0, and the remaining '.'
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denote that no following bytes have been touched. Note that, real values are
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only shown if the kernel was booted with ``no_hash_pointers``; to avoid
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information disclosure otherwise, '!' is used instead to denote invalidly
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written bytes.
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And finally, KFENCE may also report on invalid accesses to any protected page
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where it was not possible to determine an associated object, e.g. if adjacent
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object pages had not yet been allocated::
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==================================================================
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BUG: KFENCE: invalid read in test_invalid_access+0x26/0xe0
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Invalid read at 0xffffffffb670b00a:
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test_invalid_access+0x26/0xe0
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kunit_try_run_case+0x51/0x85
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kunit_generic_run_threadfn_adapter+0x16/0x30
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kthread+0x137/0x160
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ret_from_fork+0x22/0x30
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CPU: 4 PID: 124 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
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==================================================================
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DebugFS interface
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~~~~~~~~~~~~~~~~~
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Some debugging information is exposed via debugfs:
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* The file ``/sys/kernel/debug/kfence/stats`` provides runtime statistics.
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* The file ``/sys/kernel/debug/kfence/objects`` provides a list of objects
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allocated via KFENCE, including those already freed but protected.
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Implementation Details
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----------------------
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Guarded allocations are set up based on the sample interval. After expiration
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of the sample interval, the next allocation through the main allocator (SLAB or
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SLUB) returns a guarded allocation from the KFENCE object pool (allocation
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sizes up to PAGE_SIZE are supported). At this point, the timer is reset, and
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the next allocation is set up after the expiration of the interval.
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When using ``CONFIG_KFENCE_STATIC_KEYS=y``, KFENCE allocations are "gated"
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through the main allocator's fast-path by relying on static branches via the
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static keys infrastructure. The static branch is toggled to redirect the
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allocation to KFENCE. Depending on sample interval, target workloads, and
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system architecture, this may perform better than the simple dynamic branch.
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Careful benchmarking is recommended.
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KFENCE objects each reside on a dedicated page, at either the left or right
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page boundaries selected at random. The pages to the left and right of the
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object page are "guard pages", whose attributes are changed to a protected
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state, and cause page faults on any attempted access. Such page faults are then
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intercepted by KFENCE, which handles the fault gracefully by reporting an
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out-of-bounds access, and marking the page as accessible so that the faulting
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code can (wrongly) continue executing (set ``panic_on_warn`` to panic instead).
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To detect out-of-bounds writes to memory within the object's page itself,
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KFENCE also uses pattern-based redzones. For each object page, a redzone is set
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up for all non-object memory. For typical alignments, the redzone is only
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required on the unguarded side of an object. Because KFENCE must honor the
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cache's requested alignment, special alignments may result in unprotected gaps
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on either side of an object, all of which are redzoned.
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The following figure illustrates the page layout::
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---+-----------+-----------+-----------+-----------+-----------+---
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| xxxxxxxxx | O : | xxxxxxxxx | : O | xxxxxxxxx |
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| xxxxxxxxx | B : | xxxxxxxxx | : B | xxxxxxxxx |
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| x GUARD x | J : RED- | x GUARD x | RED- : J | x GUARD x |
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| xxxxxxxxx | E : ZONE | xxxxxxxxx | ZONE : E | xxxxxxxxx |
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| xxxxxxxxx | C : | xxxxxxxxx | : C | xxxxxxxxx |
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| xxxxxxxxx | T : | xxxxxxxxx | : T | xxxxxxxxx |
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---+-----------+-----------+-----------+-----------+-----------+---
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Upon deallocation of a KFENCE object, the object's page is again protected and
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the object is marked as freed. Any further access to the object causes a fault
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and KFENCE reports a use-after-free access. Freed objects are inserted at the
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tail of KFENCE's freelist, so that the least recently freed objects are reused
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first, and the chances of detecting use-after-frees of recently freed objects
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is increased.
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If pool utilization reaches 75% (default) or above, to reduce the risk of the
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pool eventually being fully occupied by allocated objects yet ensure diverse
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coverage of allocations, KFENCE limits currently covered allocations of the
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same source from further filling up the pool. The "source" of an allocation is
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based on its partial allocation stack trace. A side-effect is that this also
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limits frequent long-lived allocations (e.g. pagecache) of the same source
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filling up the pool permanently, which is the most common risk for the pool
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becoming full and the sampled allocation rate dropping to zero. The threshold
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at which to start limiting currently covered allocations can be configured via
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the boot parameter ``kfence.skip_covered_thresh`` (pool usage%).
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Interface
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---------
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The following describes the functions which are used by allocators as well as
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page handling code to set up and deal with KFENCE allocations.
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.. kernel-doc:: include/linux/kfence.h
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:functions: is_kfence_address
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kfence_shutdown_cache
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kfence_alloc kfence_free __kfence_free
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kfence_ksize kfence_object_start
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kfence_handle_page_fault
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Related Tools
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-------------
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In userspace, a similar approach is taken by `GWP-ASan
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<http://llvm.org/docs/GwpAsan.html>`_. GWP-ASan also relies on guard pages and
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a sampling strategy to detect memory unsafety bugs at scale. KFENCE's design is
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directly influenced by GWP-ASan, and can be seen as its kernel sibling. Another
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similar but non-sampling approach, that also inspired the name "KFENCE", can be
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found in the userspace `Electric Fence Malloc Debugger
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<https://linux.die.net/man/3/efence>`_.
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In the kernel, several tools exist to debug memory access errors, and in
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particular KASAN can detect all bug classes that KFENCE can detect. While KASAN
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is more precise, relying on compiler instrumentation, this comes at a
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performance cost.
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It is worth highlighting that KASAN and KFENCE are complementary, with
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different target environments. For instance, KASAN is the better debugging-aid,
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where test cases or reproducers exists: due to the lower chance to detect the
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error, it would require more effort using KFENCE to debug. Deployments at scale
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that cannot afford to enable KASAN, however, would benefit from using KFENCE to
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discover bugs due to code paths not exercised by test cases or fuzzers.
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