| Message ID | 20201104011406.598487-1-mpe@ellerman.id.au |
|---|---|
| State | New |
| Headers | show |
| Series | [4.19] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race | expand |
On Wed, Nov 04, 2020 at 12:14:06PM +1100, Michael Ellerman wrote: > 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(-) Now queued up, thanks! greg k-h
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);