linux-kernel - Re: [GIT PULL] sigqueue cache fix

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Message-ID: <YNlcgryyawTxPz//@gmail.com>
Date:   Mon, 28 Jun 2021 07:22:10 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] sigqueue cache fix


* Ingo Molnar <mingo@...nel.org> wrote:

>  - Producer <-> consumer: this is the most interesting race, and I think 
>    it's unsafe in theory, because the producer doesn't make sure that any 
>    previous writes to the actual queue entry (struct sigqueue *q) have 
>    reached storage before the new 'free' entry is advertised to consumers.
> 
>    So in principle CPU#0 could see a new sigqueue entry and use it, before 
>    it's fully freed.
> 
>    In *practice* it's probably safe by accident (or by undocumented 
>    intent), because there's an atomic op we have shortly before putting the 
>    queue entry into the sigqueue_cache, in __sigqueue_free():
> 
>          if (atomic_dec_and_test(&q->user->sigpending))
>                 free_uid(q->user);
> 
>    And atomic_dec_and_test() implies a full barrier - although I haven't 
>    found the place where we document it and 
>    Documentation/memory-ordering.txt is silent on it. We should probably 
>    fix that too.
> 
> At minimum the patch adding the ->sigqueue_cache should include a 
> well-documented race analysis firmly documenting the implicit barrier after 
> the atomic_dec_and_test().

I just realized that even with that implicit full barrier it's not safe: 
the producer uses q->user after the atomic_dec_and_test(). That access is 
not serialized with the later write to ->sigqueue_cache - and another CPU 
might see that entry and use the ->sigqueue_cache and corrupt q->user ...

So I think this code might have a real race on LL/SC platforms and we'll 
need an smp_mb() in sigqueue_cache_or_free()?

Thanks,

	Ingo