linux-kernel - Re: [PATCH] ipc/mqueue: Avoid relying on a stack reference past its expiry

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Date:   Wed, 5 May 2021 17:36:37 +0200
From:   Varad Gautam <varad.gautam@...e.com>
To:     Davidlohr Bueso <dbueso@...e.de>
CC:     linux-kernel@...r.kernel.org,
        Matthias von Faber <matthias.vonfaber@...-tech.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Manfred Spraul <manfred@...orfullife.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ipc/mqueue: Avoid relying on a stack reference past its
 expiry

On 5/5/21 5:11 PM, Davidlohr Bueso wrote:
> On 2021-05-05 00:49, Varad Gautam wrote:
>> The race here really is about the lifetime of __pipelined_op's `this`
>> argument only
>> being guaranteed for some duration of the call (ie, until someone sets
>> ->state = STATE_READY). It is not about when wake_q addition happens,
>> as long as it is
>> being fed a valid task_struct.
> 
> Again, it's all about ensuring that the READY_STATE is set last, the blocked
> task has no business returning in the first place, making both races (exit and
> the one reported here) similar by ending up using bogus memory.
> 
> ...
> 
>> I considered that initially, but given that the race isn't connected
>> with wakeup, I
>> preferred the current approach which makes this fact explicit by showing what's
>> valid/invalid during __pipelined_op.
> 
> I understand your point, but this is why I updated the ordering comments. Furthermore
> there is no reason to decouple the task's refcount with the wake_q_add operation, it
> just makes the code weird and harder to follow.
> 

Different tastes I guess - I'd avoid an additional case to account for in the
MQ_BARRIER comment block and have the __pipelined_op code describe itself,
which avoids some back and forth while reading.

If you're still unconvinced, I'll send out a v2 w/ wake_q_add called before
smp_store_release.

Thanks,
Varad

> Thanks,
> Davidlohr
> 

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