Patch "mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race" has been added to the 5.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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 5.9-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-5.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 8469091364ccc3e9cba51aeee3bff547a03b4c86
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
    
    [ 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@xxxxxxxxx>
    Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
    Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@xxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/Kconfig b/arch/Kconfig
index af14a567b493f..94821e3f94d16 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
 
diff --git a/fs/exec.c b/fs/exec.c
index 07910f5032e74..3622681489864 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1131,11 +1131,24 @@ 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);