Merge branch 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull umount fix from Al Viro:
 "Brown paperbag time: dumb braino in the series that went into 5.7
  broke the 'don't step into ->d_weak_revalidate() when umount(2) looks
  the victim up' behaviour.

  Spotted only now - saw

        if (!err && unlikely(nd->flags & LOOKUP_MOUNTPOINT)) {
                err = handle_lookup_down(nd);
                nd->flags &= ~LOOKUP_JUMPED; // no d_weak_revalidate(), please...
        }

  and went "why do we clear that flag here - nothing below that point is
  going to check it anyway" / "wait a minute, what is it doing *after*
  complete_walk() (which is where we check that flag and call
  ->d_weak_revalidate())" / "how could that possibly _not_ break?",
  followed by reproducing the breakage and verifying that the obvious
  fix of that braino does, indeed, fix it.

  The reproducer is (assuming that $DIR exists and is exported r/w to
  localhost)

      mkdir $DIR/a
      mkdir /tmp/foo
      mount --bind /tmp/foo /tmp/foo
      mkdir /tmp/foo/a
      mkdir /tmp/foo/b
      mount -t nfs4 localhost:$DIR/a /tmp/foo/a
      mount -t nfs4 localhost:$DIR /tmp/foo/b
      rmdir /tmp/foo/b/a
      umount /tmp/foo/b
      umount /tmp/foo/a
      umount -l /tmp/foo      # will get everything under /tmp/foo, no matter what

  Correct behaviour is successful umount; broken kernels (5.7-rc1 and
  later) get

      umount.nfs4: /tmp/foo/a: Stale file handle

  Note that bind mount is there to be able to recover - on broken
  kernels we'd get stuck with impossible-to-umount filesystem if not for
  that.

  FWIW, that braino had been posted for review back then, at least
  twice. Unfortunately, the call of complete_walk() was outside of diff
  context, so the bogosity hadn't been immediately obvious from the
  patch alone ;-/"

* 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  LOOKUP_MOUNTPOINT: we are cleaning "jumped" flag too late

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/namei.c b/fs/namei.c
index fc8760d4314e2..48a2f288e8023 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2421,16 +2421,16 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path
 	while (!(err = link_path_walk(s, nd)) &&
 	       (s = lookup_last(nd)) != NULL)
 		;
+	if (!err && unlikely(nd->flags & LOOKUP_MOUNTPOINT)) {
+		err = handle_lookup_down(nd);
+		nd->flags &= ~LOOKUP_JUMPED; // no d_weak_revalidate(), please...
+	}
 	if (!err)
 		err = complete_walk(nd);
 
 	if (!err && nd->flags & LOOKUP_DIRECTORY)
 		if (!d_can_lookup(nd->path.dentry))
 			err = -ENOTDIR;
-	if (!err && unlikely(nd->flags & LOOKUP_MOUNTPOINT)) {
-		err = handle_lookup_down(nd);
-		nd->flags &= ~LOOKUP_JUMPED; // no d_weak_revalidate(), please...
-	}
 	if (!err) {
 		*path = nd->path;
 		nd->path.mnt = NULL;