linux-kernel - Re: [PATCH] Detect early free of a live mm

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Message-ID: <20180302102453.4jhp6cmuqr4yodr5@lakrids.cambridge.arm.com>
Date:   Fri, 2 Mar 2018 10:24:54 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michal Hocko <mhocko@...e.com>,
        Peter Zijlstra <peterz@...raded.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 05:16:09PM -0800, Andrew Morton wrote:
> On Wed, 28 Feb 2018 12:14:58 +0000 Mark Rutland <mark.rutland@....com> 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.
> 
> Presumably the results usually aren't disastrous. 

It seems so.

> But they will be if we go and add BUG_ON()s!  Can we make this
> WARN_ON[_ONCE]()?  We should still get the same info from testers.

I fuzz the kernel with panic_on_warn=1, so that's practically the same
for me.

I'll respin to that effect.

Thanks,
Mark.