diff options
author | Nicholas Piggin <npiggin@gmail.com> | 2020-09-14 07:52:16 +0300 |
---|---|---|
committer | Michael Ellerman <mpe@ellerman.id.au> | 2020-09-16 05:24:31 +0300 |
commit | d53c3dfb23c45f7d4f910c3a3ca84bf0a99c6143 (patch) | |
tree | 759c7c734e0779cfc9f5172352eade914aa73816 /fs/exec.c | |
parent | d012a7190fc1fd72ed48911e77ca97ba4521bccd (diff) |
mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race
Reading and modifying current->mm and current->active_mm and switching
mm should be done with irqs off, to prevent races seeing an intermediate
state.
This is similar to commit 38cf307c1f20 ("mm: fix kthread_use_mm() vs TLB
invalidate"). At exec-time when the new mm is activated, the old one
should usually be single-threaded and no longer used, unless something
else is holding an mm_users reference (which may be possible).
Absent other mm_users, there is also a race with preemption and lazy tlb
switching. Consider the kernel_execve case where the current thread is
using a lazy tlb active mm:
call_usermodehelper()
kernel_execve()
old_mm = current->mm;
active_mm = current->active_mm;
*** preempt *** --------------------> schedule()
prev->active_mm = NULL;
mmdrop(prev active_mm);
...
<-------------------- schedule()
current->mm = mm;
current->active_mm = mm;
if (!old_mm)
mmdrop(active_mm);
If we switch back to the kernel thread from a different mm, there is a
double free of the old active_mm, and a missing free of the new one.
Closing this race only requires interrupts to be disabled while ->mm
and ->active_mm are being switched, but the TLB problem requires also
holding interrupts off over activate_mm. Unfortunately not all archs
can do that yet, e.g., arm defers the switch if irqs are disabled and
expects finish_arch_post_lock_switch() to be called to complete the
flush; um takes a blocking lock in activate_mm().
So as a first step, disable interrupts across the mm/active_mm updates
to close the lazy tlb preempt race, and provide an arch option to
extend that to activate_mm which allows architectures doing IPI based
TLB shootdowns to close the second race.
This is a bit ugly, but in the interest of fixing the bug and backporting
before all architectures are converted this is a compromise.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@gmail.com
Diffstat (limited to 'fs/exec.c')
-rw-r--r-- | fs/exec.c | 17 |
1 files changed, 15 insertions, 2 deletions
diff --git a/fs/exec.c b/fs/exec.c index a91003e28eaa..d4fb18baf1fb 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1130,11 +1130,24 @@ static int exec_mmap(struct mm_struct *mm) } task_lock(tsk); - active_mm = tsk->active_mm; membarrier_exec_mmap(mm); - tsk->mm = mm; + + local_irq_disable(); + active_mm = tsk->active_mm; tsk->active_mm = mm; + tsk->mm = mm; + /* + * This prevents preemption while active_mm is being loaded and + * it and mm are being updated, which could cause problems for + * lazy tlb mm refcounting when these are updated by context + * switches. Not all architectures can handle irqs off over + * activate_mm yet. + */ + if (!IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM)) + local_irq_enable(); activate_mm(active_mm, mm); + if (IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM)) + local_irq_enable(); tsk->mm->vmacache_seqnum = 0; vmacache_flush(tsk); task_unlock(tsk); |