This is a note to let you know that I've just added the patch titled
mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race
to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-fix-exec-activate_mm-vs-tlb-shootdown-and-lazy-tl.patch
and it can be found in the queue-4.14 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6743ee4f371a583700264efca0504a5b2b4437cd
Author: Nicholas Piggin <npiggin@xxxxxxxxx>
Date: Mon Sep 14 14:52:16 2020 +1000
mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race
commit d53c3dfb23c45f7d4f910c3a3ca84bf0a99c6143 upstream.
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@xxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
[mpe: Manual backport to 4.19 due to membarrier_exec_mmap(mm) changes]
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
diff --git a/arch/Kconfig b/arch/Kconfig
index 77b3e21c4844e..95567f6832752 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -339,6 +339,13 @@ config HAVE_RCU_TABLE_FREE
config HAVE_RCU_TABLE_INVALIDATE
bool
+config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
+ bool
+ help
+ Temporary select until all architectures can be converted to have
+ irqs disabled over activate_mm. Architectures that do IPI based TLB
+ shootdowns should enable this.
+
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
diff --git a/fs/exec.c b/fs/exec.c
index 92fd21f0b0ff7..e8d1e67059775 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1024,10 +1024,23 @@ static int exec_mmap(struct mm_struct *mm)
}
}
task_lock(tsk);
+
+ local_irq_disable();
active_mm = tsk->active_mm;
- tsk->mm = 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);
