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    Subject[PATCH 5.9 048/391] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race
    Date
    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 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);
    --
    2.27.0


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