• Nicholas Piggin's avatar
    mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race · d53c3dfb
    Nicholas Piggin authored
    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 38cf307c ("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: default avatarNicholas Piggin <npiggin@gmail.com>
    Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@gmail.com
    d53c3dfb
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