Commit 250cf369 authored Apr 27, 2025 by Al Viro

__legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock



... or we risk stealing final mntput from sync umount - raising mnt_count
after umount(2) has verified that victim is not busy, but before it
has set MNT_SYNC_UMOUNT; in that case __legitimize_mnt() doesn't see
that it's safe to quietly undo mnt_count increment and leaves dropping
the reference to caller, where it'll be a full-blown mntput().

Check under mount_lock is needed; leaving the current one done before
taking that makes no sense - it's nowhere near common enough to bother
with.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent 92a09c47

fs/namespace.c

+1 −5

Original line number	Diff line number	Diff line
		@@ -790,12 +790,8 @@ int __legitimize_mnt(struct vfsmount *bastard, unsigned seq)
		smp_mb(); // see mntput_no_expire()
		if (likely(!read_seqretry(&mount_lock, seq)))
		return 0;
		if (bastard->mnt_flags & MNT_SYNC_UMOUNT) {
		mnt_add_count(mnt, -1);
		return 1;
		}
		lock_mount_hash();
		if (unlikely(bastard->mnt_flags & MNT_DOOMED)) {
		if (unlikely(bastard->mnt_flags & (MNT_SYNC_UMOUNT \| MNT_DOOMED))) {
		mnt_add_count(mnt, -1);
		unlock_mount_hash();
		return 1;