linux-kernel - Re: arm64/v4.16-rc1: KASAN: use-after-free Read in finish_task

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Date:   Thu, 15 Feb 2018 13:13:39 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: arm64/v4.16-rc1: KASAN: use-after-free Read in
 finish_task_switch

----- On Feb 15, 2018, at 6:49 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Wed, Feb 14, 2018 at 06:53:44PM +0000, Mathieu Desnoyers wrote:
>> However, given the scenario involves multiples CPUs (one doing exit_mm(),
>> the other doing context switch), the actual order of perceived load/store
>> can be shuffled. And AFAIU nothing prevents the CPU from ordering the
>> atomic_inc() done by mmgrab(mm) _after_ the store to current->mm.
>> 
>> I wonder if we should not simply add a smp_mb__after_atomic() into
>> mmgrab() instead ? I see that e.g. futex.c does:
> 
> Don't think so, the futex case is really rather special and I suspect
> this one is too. I would much rather have explicit comments rather than
> implicit works by magic.
> 
> As per the rationale used for refcount_t, increments should be
> unordered, because you ACQUIRE your object _before_ you can do the
> increment.
> 
> The futex thing is simply abusing a bunch of implied barriers and
> patching up the holes in paths that didn't already imply a barrier in
> order to avoid having to add explicit barriers (which had measurable
> performance issues).
> 
> And here we have explicit ordering outside of the reference counting
> too, we want to ensure the reference is incremented before we modify
> a second object.
> 
> This ordering is not at all related to acquiring the reference, so
> bunding it seems odd.

I understand your point. Will's added barrier and comment is fine.

Thanks,

Mathieu



-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com