linux-kernel - Re: [PATCH -next v2] locking/percpu-rwsem: fix a task

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Date:   Mon, 6 Apr 2020 16:09:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        dbueso@...e.de, juri.lelli@...hat.com,
        Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] locking/percpu-rwsem: fix a task_struct refcount

On Mon, Apr 06, 2020 at 09:31:18AM -0400, Qian Cai wrote:
> 
> 
> > On Mar 30, 2020, at 5:30 PM, Qian Cai <cai@....pw> wrote:
> > 
> > The commit 7f26482a872c ("locking/percpu-rwsem: Remove the embedded
> > rwsem") introduced some task_struct memory leaks due to messing up with
> > a task_struct refcount. At the beginning of
> > percpu_rwsem_wake_function(), it calls get_task_struct(), but if the
> > trylock failed, it will remain in the waitqueue. However, it will run
> > percpu_rwsem_wake_function() again with get_task_struct() to increase
> > the refcount but then only call put_task_struct() once the trylock
> > succeeded.
> > 
> > Fix it by adjusting percpu_rwsem_wake_function() a bit to guard against
> > when percpu_rwsem_wait() observing !private, terminating the wait and
> > doing a quick exit() while percpu_rwsem_wake_function() then doing
> > wake_up_process(p) as a use-after-free.
> > 
> > Fixes: 7f26482a872c ("locking/percpu-rwsem: Remove the embedded rwsem")
> 
> Peter, Ingo, can you take a look at this patch when you have a chance?

I have it queued for urgent, should probably hit tip soon.