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

David J. Wilder
2007-07-19T19:23:09Z

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@gmail.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@us.ibm.com), IBM Corp
> * Copyright (C) 1999-2005 - Karim Yaghmour (karim@opersys.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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