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

Mathieu Desnoyers
2007-07-20T01:09:21Z

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.

Note from Mathieu: the documentation reference change should have been
done in a separate patch, but I guess no one will really care.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Acked-by: "David J. Wilder" <wilder@us.ibm.com>
Tested-by: "David J. Wilder" <wilder@us.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---

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:
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68