ceph: fix potential use-after-free in ceph_mdsc_build_path

temp is not defined outside of the RCU critical section here. Ensure
we grab that value before we drop the rcu_read_lock.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 761cb669aa13..b01e2043b1b2 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -2089,13 +2089,14 @@ static inline  u64 __get_oldest_tid(struct ceph_mds_client *mdsc)
  * Encode hidden .snap dirs as a double /, i.e.
  *   foo/.snap/bar -> foo//bar
  */
-char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
+char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
                            int stop_on_nosnap)
 {
         struct dentry *temp;
         char *path;
         int len, pos;
         unsigned seq;
+        u64 base;
 
         if (!dentry)
                 return ERR_PTR(-EINVAL);
@@ -2151,6 +2152,7 @@ retry:
                         path[--pos] = '/';
                 temp = temp->d_parent;
         }
+        base = ceph_ino(d_inode(temp));
         rcu_read_unlock();
         if (pos != 0 || read_seqretry(&rename_lock, seq)) {
                 pr_err("build_path did not end path lookup where "
@@ -2163,10 +2165,10 @@ retry:
                 goto retry;
         }
 
-        *base = ceph_ino(d_inode(temp));
+        *pbase = base;
         *plen = len;
         dout("build_path on %p %d built %llx '%.*s'\n",
-             dentry, d_count(dentry), *base, len, path);
+             dentry, d_count(dentry), base, len, path);
         return path;
 }