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__legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

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... 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>
This commit is contained in:
Al Viro
2025-04-27 15:41:51 -04:00
parent 92a09c4746
commit 250cf36930
1 changed files with 1 additions and 5 deletions
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6
fs/namespace.c
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@@ -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;