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

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Message-ID: <20180301093522.GC15057@dhcp22.suse.cz>
Date:   Thu, 1 Mar 2018 10:35:22 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
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 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));

Relying on current->mm resetting works currently but this is quite a
subtle dependency.
-- 
Michal Hocko
SUSE Labs