linux-kernel - Re: [PATCH] Fix a use after free bug in kernel->userspace relay file support

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Date:	Thu, 19 Jul 2007 12:22:36 -0700
From:	"David J. Wilder" <wilder@...ibm.com>
To:	Jesper Juhl <jesper.juhl@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Karim Yaghmour <karim@...rsys.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH] Fix a use after free bug in kernel->userspace relay file
 support

ACK
Thanks for catching this. Your patch looks fine. I tested for 
regression, no problems. I also tested the error path and had the 
expected results.
Thanks
Dave

Jesper Juhl wrote:
> Hi,
>
> Coverity spotted what looks like a real possible case of using a 
> variable after it has been freed.
> The problem is in kernel/relay.c::relay_open_buf()
>
> If the code hits "goto free_buf;" it ends up in this code :
>
>   free_buf:
>     	relay_destroy_buf(buf);	<--- calls kfree() on 'buf'.
>   free_name:
>    	kfree(tmpname);
>   end:
>   	return buf;		<-- use after free of 'buf'.
>
> I read through the callers and they all handle a NULL return 
> from this function as an error (and hitting the 'free_buf' label 
> only happens on failure to chan->cb->create_buf_file(), so that 
> looks like a clear error to me). 
>
> The patch simply sets 'buf' to NULL after the call to 
> relay_destroy_buf(buf); - as far as I can see that should take 
> care of the problem.
>
> The patch also corrects a reference to a documentation file while 
> I was at it.
>
> Warning: I don't know this code at all and I've only read through 
> it quickly to try and work out what to do here, so I'd really 
> apreciate it if someone who actually knows how this code should 
> behave could ACK or NACK the patch before it gets merged anywhere 
> since I did this pretty blind.
>
> Patch has been compile tested only.
>
>
> Signed-off-by: Jesper Juhl <jesper.juhl@...il.com>
> ---
>
> diff --git a/kernel/relay.c b/kernel/relay.c
> index a615a8f..c55e399 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -1,7 +1,7 @@
>  /*
>   * Public API and common code for kernel->userspace relay file support.
>   *
> - * See Documentation/filesystems/relayfs.txt for an overview of relayfs.
> + * See Documentation/filesystems/relay.txt for an overview.
>   *
>   * Copyright (C) 2002-2005 - Tom Zanussi (zanussi@...ibm.com), IBM Corp
>   * Copyright (C) 1999-2005 - Karim Yaghmour (karim@...rsys.com)
> @@ -427,6 +427,7 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>
>  free_buf:
>   	relay_destroy_buf(buf);
> + 	buf = NULL;
>  free_name:
>   	kfree(tmpname);
>  end:
>
>
> -
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