rcuwalk: recheck mount_lock after mountpoint crossing attempts

We can get false negative from __lookup_mnt() if an unrelated vfsmount
gets moved.  In that case legitimize_mnt() is guaranteed to fail,
and we will fall back to non-RCU walk... unless we end up running
into a hard error on a filesystem object we wouldn't have reached
if not for that false negative.  IOW, delaying that check until
the end of pathname resolution is wrong - we should recheck right
after we attempt to cross the mountpoint.  We don't need to recheck
unless we see d_mountpoint() being true - in that case even if
we have just raced with mount/umount, we can simply go on as if
we'd come at the moment when the sucker wasn't a mountpoint; if we
run into a hard error as the result, it was a legitimate outcome.
__lookup_mnt() returning NULL is different in that respect, since
it might've happened due to operation on completely unrelated
mountpoint.

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

1 files changed, 13 insertions, 16 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 2f730ef9b4b3..4b491b431990 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1109,7 +1109,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                         return false;
 
                 if (!d_mountpoint(path->dentry))
-                        break;
+                        return true;
 
                 mounted = __lookup_mnt(path->mnt, path->dentry);
                 if (!mounted)
@@ -1125,20 +1125,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
                  */
                 *inode = path->dentry->d_inode;
         }
-        return true;
-}
-
-static void follow_mount_rcu(struct nameidata *nd)
-{
-        while (d_mountpoint(nd->path.dentry)) {
-                struct mount *mounted;
-                mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
-                if (!mounted)
-                        break;
-                nd->path.mnt = &mounted->mnt;
-                nd->path.dentry = mounted->mnt.mnt_root;
-                nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
-        }
+        return read_seqretry(&mount_lock, nd->m_seq);
 }
 
 static int follow_dotdot_rcu(struct nameidata *nd)
@@ -1166,7 +1153,17 @@ static int follow_dotdot_rcu(struct nameidata *nd)
                         break;
                 nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
         }
-        follow_mount_rcu(nd);
+        while (d_mountpoint(nd->path.dentry)) {
+                struct mount *mounted;
+                mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
+                if (!mounted)
+                        break;
+                nd->path.mnt = &mounted->mnt;
+                nd->path.dentry = mounted->mnt.mnt_root;
+                nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
+                if (!read_seqretry(&mount_lock, nd->m_seq))
+                        goto failed;
+        }
         nd->inode = nd->path.dentry->d_inode;
         return 0;