From: Nicholas Piggin npiggin@gmail.com

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@gmail.com Acked-by: Peter Zijlstra (Intel) peterz@infradead.org [mpe: Manual backport to 4.19 due to membarrier_exec_mmap(mm) changes] Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@gmail.com --- arch/Kconfig | 7 +++++++ fs/exec.c | 15 ++++++++++++++- 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig index a336548487e6..e3a030f7a722 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -366,6 +366,13 @@ 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 cece8c14f377..52788644c4af 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1028,10 +1028,23 @@ 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);