namei: ->d_inode of a pinned dentry is stable only for positives

both do_last() and walk_component() risk picking a NULL inode out
of dentry about to become positive, *then* checking its flags and
seeing that it's not negative anymore and using (already stale by
then) value they'd fetched earlier.  Usually ends up oopsing soon
after that...

Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index e30deef..e0881c0 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1743,11 +1743,11 @@ static int walk_component(struct nameidata *nd, int flags)
                if (err < 0)
                        return err;
 
-               inode = d_backing_inode(path.dentry);
                seq = 0;        /* we are already out of RCU mode */
                err = -ENOENT;
                if (d_is_negative(path.dentry))
                        goto out_path_put;
+               inode = d_backing_inode(path.dentry);
        }
 
        if (flags & WALK_PUT)
@@ -3192,12 +3192,12 @@ retry_lookup:
                return error;
 
        BUG_ON(nd->flags & LOOKUP_RCU);
-       inode = d_backing_inode(path.dentry);
        seq = 0;        /* out of RCU mode, so the value doesn't matter */
        if (unlikely(d_is_negative(path.dentry))) {
                path_to_nameidata(&path, nd);
                return -ENOENT;
        }
+       inode = d_backing_inode(path.dentry);
 finish_lookup:
        if (nd->depth)
                put_link(nd);