vfs: fix race in rcu lookup of pruned dentry

Don't update *inode in __follow_mount_rcu() until we'd verified that
there is mountpoint there.  Kudos to Hugh Dickins for catching that
one in the first place and eventually figuring out the solution (and
catching a braino in the earlier version of patch).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 5c867dd..14ab8d3 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -942,7 +942,6 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                 * Don't forget we might have a non-mountpoint managed dentry
                 * that wants to block transit.
                 */
-               *inode = path->dentry->d_inode;
                if (unlikely(managed_dentry_might_block(path->dentry)))
                        return false;
 
@@ -955,6 +954,12 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                path->mnt = mounted;
                path->dentry = mounted->mnt_root;
                nd->seq = read_seqcount_begin(&path->dentry->d_seq);
+               /*
+                * Update the inode too. We don't need to re-check the
+                * dentry sequence number here after this d_inode read,
+                * because a mount-point is always pinned.
+                */
+               *inode = path->dentry->d_inode;
        }
        return true;
 }