Merge tag 'fs.fixes.v5.18-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

Pull mount_setattr fix from Christian Brauner:
 "The recent cleanup in e257039f0fc7 ("mount_setattr(): clean the
  control flow and calling conventions") switched the mount attribute
  codepaths from do-while to for loops as they are more idiomatic when
  walking mounts.

  However, we did originally choose do-while constructs because if we
  request a mount or mount tree to be made read-only we need to hold
  writers in the following way: The mount attribute code will grab
  lock_mount_hash() and then call mnt_hold_writers() which will
  _unconditionally_ set MNT_WRITE_HOLD on the mount.

  Any callers that need write access have to call mnt_want_write(). They
  will immediately see that MNT_WRITE_HOLD is set on the mount and the
  caller will then either spin (on non-preempt-rt) or wait on
  lock_mount_hash() (on preempt-rt).

  The fact that MNT_WRITE_HOLD is set unconditionally means that once
  mnt_hold_writers() returns we need to _always_ pair it with
  mnt_unhold_writers() in both the failure and success paths.

  The do-while constructs did take care of this. But Al's change to a
  for loop in the failure path stops on the first mount we failed to
  change mount attributes _without_ going into the loop to call
  mnt_unhold_writers().

  This in turn means that once we failed to make a mount read-only via
  mount_setattr() - i.e. there are already writers on that mount - we
  will block any writers indefinitely. Fix this by ensuring that the for
  loop always unsets MNT_WRITE_HOLD including the first mount we failed
  to change to read-only. Also sprinkle a few comments into the cleanup
  code to remind people about what is happening including myself. After
  all, I didn't catch it during review.

  This is only relevant on mainline and was reported by syzbot. Details
  about the syzbot reports are all in the commit message"

* tag 'fs.fixes.v5.18-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  fs: unset MNT_WRITE_HOLD on failure

1 files changed, 13 insertions, 1 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index a0a36bfa3aa0..afe2b64b14f1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4058,10 +4058,22 @@ static int mount_setattr_prepare(struct mount_kattr *kattr, struct mount *mnt)
 	if (err) {
 		struct mount *p;
 
-		for (p = mnt; p != m; p = next_mnt(p, mnt)) {
+		/*
+		 * If we had to call mnt_hold_writers() MNT_WRITE_HOLD will
+		 * be set in @mnt_flags. The loop unsets MNT_WRITE_HOLD for all
+		 * mounts and needs to take care to include the first mount.
+		 */
+		for (p = mnt; p; p = next_mnt(p, mnt)) {
 			/* If we had to hold writers unblock them. */
 			if (p->mnt.mnt_flags & MNT_WRITE_HOLD)
 				mnt_unhold_writers(p);
+
+			/*
+			 * We're done once the first mount we changed got
+			 * MNT_WRITE_HOLD unset.
+			 */
+			if (p == m)
+				break;
 		}
 	}
 	return err;