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tracing/user_events: Limit max fault-in attempts

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When event enablement changes, user_events attempts to update a bit in
the user process. If a fault is hit, an attempt to fault-in the page and
the write is retried if the page made it in. While this normally requires
a couple attempts, it is possible a bad user process could attempt to
cause infinite loops.

Ensure fault-in attempts either sync or async are limited to a max of 10
attempts for each update. When the max is hit, return -EFAULT so another
attempt is not made in all cases.

Link: https://lkml.kernel.org/r/20230425225107.8525-5-beaub@linux.microsoft.com

Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Beau Belgrave 2023-04-25 15:51:07 -07:00 committed by Steven Rostedt (Google)
parent 97bbce89bf
commit 41d8fba193
1 changed files with 35 additions and 14 deletions
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49
kernel/trace/trace_events_user.c
View File

@ -123,6 +123,7 @@ struct user_event_enabler_fault {
struct work_struct work;
struct user_event_mm *mm;
struct user_event_enabler *enabler;
int attempt;
};
static struct kmem_cache *fault_cache;
@ -266,11 +267,19 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler)
kfree(enabler);
}
static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr)
static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr,
int attempt)
{
bool unlocked;
int ret;
/*
* Normally this is low, ensure that it cannot be taken advantage of by
* bad user processes to cause excessive looping.
*/
if (attempt > 10)
return -EFAULT;
mmap_read_lock(mm->mm);
/* Ensure MM has tasks, cannot use after exit_mm() */
@ -289,7 +298,7 @@ out:
static int user_event_enabler_write(struct user_event_mm *mm,
struct user_event_enabler *enabler,
bool fixup_fault);
bool fixup_fault, int *attempt);
static void user_event_enabler_fault_fixup(struct work_struct *work)
{
@ -298,9 +307,10 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
struct user_event_enabler *enabler = fault->enabler;
struct user_event_mm *mm = fault->mm;
unsigned long uaddr = enabler->addr;
int attempt = fault->attempt;
int ret;
ret = user_event_mm_fault_in(mm, uaddr);
ret = user_event_mm_fault_in(mm, uaddr, attempt);
if (ret && ret != -ENOENT) {
struct user_event *user = enabler->event;
@ -329,7 +339,7 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
if (!ret) {
mmap_read_lock(mm->mm);
user_event_enabler_write(mm, enabler, true);
user_event_enabler_write(mm, enabler, true, &attempt);
mmap_read_unlock(mm->mm);
}
out:
@ -341,7 +351,8 @@ out:
}
static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
struct user_event_enabler *enabler)
struct user_event_enabler *enabler,
int attempt)
{
struct user_event_enabler_fault *fault;
@ -353,6 +364,7 @@ static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
INIT_WORK(&fault->work, user_event_enabler_fault_fixup);
fault->mm = user_event_mm_get(mm);
fault->enabler = enabler;
fault->attempt = attempt;
/* Don't try to queue in again while we have a pending fault */
set_bit(ENABLE_VAL_FAULTING_BIT, ENABLE_BITOPS(enabler));
@ -372,7 +384,7 @@ static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
static int user_event_enabler_write(struct user_event_mm *mm,
struct user_event_enabler *enabler,
bool fixup_fault)
bool fixup_fault, int *attempt)
{
unsigned long uaddr = enabler->addr;
unsigned long *ptr;
@ -383,6 +395,8 @@ static int user_event_enabler_write(struct user_event_mm *mm,
lockdep_assert_held(&event_mutex);
mmap_assert_locked(mm->mm);
*attempt += 1;
/* Ensure MM has tasks, cannot use after exit_mm() */
if (refcount_read(&mm->tasks) == 0)
return -ENOENT;
@ -398,7 +412,7 @@ static int user_event_enabler_write(struct user_event_mm *mm,
if (!fixup_fault)
return -EFAULT;
if (!user_event_enabler_queue_fault(mm, enabler))
if (!user_event_enabler_queue_fault(mm, enabler, *attempt))
pr_warn("user_events: Unable to queue fault handler\n");
return -EFAULT;
@ -439,15 +453,19 @@ static void user_event_enabler_update(struct user_event *user)
struct user_event_enabler *enabler;
struct user_event_mm *mm = user_event_mm_get_all(user);
struct user_event_mm *next;
int attempt;
while (mm) {
next = mm->next;
mmap_read_lock(mm->mm);
rcu_read_lock();
list_for_each_entry_rcu(enabler, &mm->enablers, link)
if (enabler->event == user)
user_event_enabler_write(mm, enabler, true);
list_for_each_entry_rcu(enabler, &mm->enablers, link) {
if (enabler->event == user) {
attempt = 0;
user_event_enabler_write(mm, enabler, true, &attempt);
}
}
rcu_read_unlock();
mmap_read_unlock(mm->mm);
@ -695,6 +713,7 @@ static struct user_event_enabler
struct user_event_enabler *enabler;
struct user_event_mm *user_mm;
unsigned long uaddr = (unsigned long)reg->enable_addr;
int attempt = 0;
user_mm = current_user_event_mm();
@ -715,7 +734,8 @@ retry:
/* Attempt to reflect the current state within the process */
mmap_read_lock(user_mm->mm);
*write_result = user_event_enabler_write(user_mm, enabler, false);
*write_result = user_event_enabler_write(user_mm, enabler, false,
&attempt);
mmap_read_unlock(user_mm->mm);
/*
@ -735,7 +755,7 @@ retry:
if (*write_result) {
/* Attempt to fault-in and retry if it worked */
if (!user_event_mm_fault_in(user_mm, uaddr))
if (!user_event_mm_fault_in(user_mm, uaddr, attempt))
goto retry;
kfree(enabler);
@ -2195,6 +2215,7 @@ static int user_event_mm_clear_bit(struct user_event_mm *user_mm,
{
struct user_event_enabler enabler;
int result;
int attempt = 0;
memset(&enabler, 0, sizeof(enabler));
enabler.addr = uaddr;
@ -2205,14 +2226,14 @@ retry:
/* Force the bit to be cleared, since no event is attached */
mmap_read_lock(user_mm->mm);
result = user_event_enabler_write(user_mm, &enabler, false);
result = user_event_enabler_write(user_mm, &enabler, false, &attempt);
mmap_read_unlock(user_mm->mm);
mutex_unlock(&event_mutex);
if (result) {
/* Attempt to fault-in and retry if it worked */
if (!user_event_mm_fault_in(user_mm, uaddr))
if (!user_event_mm_fault_in(user_mm, uaddr, attempt))
goto retry;
}