[PATCH AUTOSEL 5.8 005/132] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

Sasha Levin
2020-10-27T00:32:08Z

From: Nicholas Piggin <npiggin@gmail.com>

[ Upstream commit d53c3dfb23c45f7d4f910c3a3ca84bf0a99c6143 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/Kconfig | 7 +++++++
fs/exec.c | 17 +++++++++++++++--
2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c72..2fb97d79e7693 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -411,6 +411,13 @@ config MMU_GATHER_NO_GATHER
bool
depends on MMU_GATHER_TABLE_FREE

+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 e6e8a9a703278..791384bb02b05 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1103,11 +1103,24 @@ 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);
--
2.25.1