linux-kernel - Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...

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Date:	Mon, 24 Sep 2007 08:54:08 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Nadia Derbey <Nadia.Derbey@...l.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...ru>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.23-rc6-mm1: IPC: sleeping function called ...

On Fri, Sep 21, 2007 at 01:03:47PM +0200, Jarek Poplawski wrote:
...
> I hope not! But, then it would be probably another logical trick:
> ipc_rcu_getref/putref() seems to prevent kfreeing of a structure, so
> if it's used in do_msgsnd() there should be a risk something can do
> this kfree at this moment, and it seems freeque() is the only one,
> which both: can do this and cares for this refcount. Then, e.g., if
> any of them does ipc_rcu_getref() a bit later and sees old (cached)
> value - kfree can be skipped forever. [...]

After rethinking, this scenario seems to be wrong or very unprobable
(I'm not sure of all ways "if (--container...)" could be compiled),
so there should be no such risk - double kfree/vfree is more probable,
so no danger. More likely is such refcount abuse: ipc_rcu_getref() in
do_msgsnd() done a bit after ipc_rcu_putref() in freeque() (msq
pointer acquired by do_msgsend() before freeque() started); then,
after schedule(), do_msgsnd() can work with kfreed msq_queue structure
(at least considering classic RCU).

Regards,
Jarek P.
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