Re: [PATCH -next] fork: silence a false postive warning in __mmdrop

[Qian Cai]

On Wed, Jul 22, 2020 at 12:06:37PM +0200, peterz@xxxxxxxxxxxxx wrote:
> On Thu, Jun 04, 2020 at 11:03:44AM -0400, Qian Cai wrote:
> > The linux-next commit bf2c59fce407 ("sched/core: Fix illegal RCU from
> > offline CPUs") delayed,
> > 
> > idle->active_mm = &init_mm;
> > 
> > into finish_cpu() instead of idle_task_exit() which results in a false
> > positive warning that was originally designed in the commit 3eda69c92d47
> > ("kernel/fork.c: detect early free of a live mm").
> > 
> >  WARNING: CPU: 127 PID: 72976 at kernel/fork.c:697
> >  __mmdrop+0x230/0x2c0
> >  do_exit+0x424/0xfa0
> >  Call Trace:
> >  do_exit+0x424/0xfa0
> >  do_group_exit+0x64/0xd0
> >  sys_exit_group+0x24/0x30
> >  system_call_exception+0x108/0x1d0
> >  system_call_common+0xf0/0x278
> 
> Please explain; because afaict this is a use-after-free.
> 
> The thing is __mmdrop() is going to actually free the mm, so then what
> is finish_cpu()'s mmdrop() going to do?
> 
> ->active_mm() should have a refcount on the mm.

Well, the refcount issue you mentioned then happens all before bf2c59fce407 was
introduced as well, but then it looks harmless because mmdrop() in finish_cpu()
will do,

	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
		__mmdrop(mm);

where that atomic_dec_and_test() see the negative refcount and will not involve
__mmdrop() again. It is not clear to me that once the CPU is offline if it
needs to care about its idle thread mm_count at all. Even if this refcount
issue is finally addressed, it could hit this warning in finish_cpu() without
this patch.

On the other hand, if you look at the commit 3eda69c92d47, it is clearly that
the assumption of,

   WARN_ON_ONCE(mm == current->active_mm);

is totally gone due to bf2c59fce407. Thus, the patch is to fix that discrepancy
first and then I'll look at that the imbalance mmdrop()/mmgrab() elsewhere.

> 
> > Fixes: bf2c59fce407 ("sched/core: Fix illegal RCU from offline CPUs")
> > Signed-off-by: Qian Cai <cai@xxxxxx>
> > ---
> >  kernel/fork.c | 1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 142b23645d82..5334efd2a680 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -694,7 +694,6 @@ void __mmdrop(struct mm_struct *mm)
> >  {
> >  	BUG_ON(mm == &init_mm);
> >  	WARN_ON_ONCE(mm == current->mm);
> > -	WARN_ON_ONCE(mm == current->active_mm);
> >  	mm_free_pgd(mm);
> >  	destroy_context(mm);
> >  	mmu_notifier_subscriptions_destroy(mm);
> > -- 
> > 2.21.0 (Apple Git-122.2)
> >