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Message-ID: <20180301124925.k6q2fxvrphu7m7m6@lakrids.cambridge.arm.com>
Date: Thu, 1 Mar 2018 12:49:26 +0000
From: Mark Rutland <mark.rutland@....com>
To: Michal Hocko <mhocko@...nel.org>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Peter Zijlstra <peterz@...radead.org>,
Rik van Riel <riel@...hat.com>,
Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] Detect early free of a live mm
On Thu, Mar 01, 2018 at 01:40:24PM +0100, Michal Hocko wrote:
> On Thu 01-03-18 11:22:37, Mark Rutland wrote:
> > On Thu, Mar 01, 2018 at 10:35:22AM +0100, Michal Hocko wrote:
> > > On Wed 28-02-18 12:18:10, Mark Rutland wrote:
> > > > Ugh, I messed up Peter's email when sending this out. please
> > > > s/infraded/infradead/ if replying to the first mail.
> > > >
> > > > Sorry about that.
> > > >
> > > > Mark.
> > > >
> > > > On Wed, Feb 28, 2018 at 12:14:58PM +0000, Mark Rutland wrote:
> > > > > KASAN splats indicate that in some cases we free a live mm, then
> > > > > continue to access it, with potentially disastrous results. This is
> > > > > likely due to a mismatched mmdrop() somewhere in the kernel, but so far
> > > > > the culprit remains elusive.
> > > > >
> > > > > Let's have __mmdrop() verify that the mm isn't live for the current
> > > > > task, similar to the existing check for init_mm. This way, we can catch
> > > > > this class of issue earlier, and without requiring KASAN.
> > >
> > > Wouldn't it be better to check the mm_users count instead?
> > > {VM_}BUG_ON(atomic_read(&mm->mm_users));
> >
> > Perhaps, but that won't catch a mismatched mmput(), which could
> > decrement mm_users (and mm_count) to zero early, before current->mm is
> > cleared in exit_mm().
>
> true
>
> > Locally, I'm testing with:
> >
> > BUG_ON(mm == current->mm);
> > BUG_ON(mm == current->active_mm);
> > BUG_ON(refcount_read(&mm->mm_users) != 0);
> > BUG_ON(refcount_read(&mm->mm_count) != 0);
> >
> > ... so as to also catch an early free from another thread.
>
> I would be careful about active_mm. At least {un}use_mm does some tricks
> with it.
If that can leave mm == current->active_mm during __mmdrop(), I think
that would be a bug in its own right, since that would leave
current->active_mm pointing at a freed mm.
AFAICT, use_mm() does the usual mm_count balancing when flipping
active_mm (i.e. we mmgrab() the new mm, make it active_mm, then mmdrop()
the old active_mm), so even if the mmdrop() ends up freeing the old
active_mm, that shouldn't trigger the BUG_ON(mm == current->active_mm).
In unuse_mm() we don't poke active_mm at all, and leave that to the
usual context_switch() + finish_task_switch() grab/drop.
> > > Relying on current->mm resetting works currently but this is quite a
> > > subtle dependency.
> >
> > Do you mean because it only catches an early free in a thread using that
> > mm, or is there another subtlety you had in mind?
>
> No, I was more thinking about future changes when we stop clearing
> tsk->mm at exit. This is rather unlikely to be honest because that
> requires more changes.
Ok.
Thanks,
Mark.
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