GFS2: Fix race in gfs2_rs_alloc

QE aio tests uncovered a race condition in gfs2_rs_alloc where it's possible
to come out of the function with a valid ip->i_res allocation but it gets
freed before use resulting in a NULL ptr dereference.

This patch envelopes the initial short-circuit check for non-NULL ip->i_res
into the mutex lock. With this patch, I was able to successfully run the
reproducer test multiple times.

Resolves: rhbz#878476
Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index 37ee061d899e81a8eb45f5bc64382634e573c108..738b3888adc639bf859e182bd9740457fed8cc70 100644 (file)
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -557,22 +557,20 @@ void gfs2_free_clones(struct gfs2_rgrpd *rgd)
  */
 int gfs2_rs_alloc(struct gfs2_inode *ip)
 {
-       struct gfs2_blkreserv *res;
+       int error = 0;
 
+       down_write(&ip->i_rw_mutex);
        if (ip->i_res)
-               return 0;
-
-       res = kmem_cache_zalloc(gfs2_rsrv_cachep, GFP_NOFS);
-       if (!res)
-               return -ENOMEM;
+               goto out;
 
-       RB_CLEAR_NODE(&res->rs_node);
+       ip->i_res = kmem_cache_zalloc(gfs2_rsrv_cachep, GFP_NOFS);
+       if (!ip->i_res) {
+               error = -ENOMEM;
+               goto out;
+       }
 
-       down_write(&ip->i_rw_mutex);
-       if (ip->i_res)
-               kmem_cache_free(gfs2_rsrv_cachep, res);
-       else
-               ip->i_res = res;
+       RB_CLEAR_NODE(&ip->i_res->rs_node);
+out:
        up_write(&ip->i_rw_mutex);
        return 0;
 }