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authorNicholas Piggin <npiggin@gmail.com>2020-09-14 14:52:16 +1000
committerMichael Ellerman <mpe@ellerman.id.au>2020-09-16 12:24:31 +1000
commitd53c3dfb23c45f7d4f910c3a3ca84bf0a99c6143 (patch)
tree759c7c734e0779cfc9f5172352eade914aa73816 /arch/Kconfig
parentd012a7190fc1fd72ed48911e77ca97ba4521bccd (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 'arch/Kconfig')
-rw-r--r--arch/Kconfig7
1 files changed, 7 insertions, 0 deletions
diff --git a/arch/Kconfig b/arch/Kconfig
index af14a567b493..94821e3f94d1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -414,6 +414,13 @@ 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