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

[Mathieu Desnoyers]

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@xxxxxxxxx>
Acked-by: "David J. Wilder" <wilder@xxxxxxxxxx>
Tested-by: "David J. Wilder" <wilder@xxxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
---

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@xxxxxxxxxx), IBM Corp
  * Copyright (C) 1999-2005 - Karim Yaghmour (karim@xxxxxxxxxxx)
@@ -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
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