CVE-2025-37988

In the Linux kernel, the following vulnerability has been resolved:

fix a couple of races in MNTTREEBENEATH handling by domovemount()

Normally dolockmount(path, ) is locking a mountpoint pinned by *path and at the time when matching unlockmount() unlocks that location it is still pinned by the same thing.

Unfortunately, for 'beneath' case it's no longer that simple - the object being locked is not the one *path points to. It's the mountpoint of path->mnt. The thing is, without sufficient locking ->mntparent may change under us and none of the locks are held at that point. The rules are * mountlock stabilizes m->mntparent for any mount m. * namespacesem stabilizes m->mntparent, provided that m is mounted. * if either of the above holds and refcount of m is positive, we are guaranteed the same for refcount of m->mntparent.

namespacesem nests inside inodelock(), so dolockmount() has to take inodelock() before grabbing namespacesem. It does recheck that path->mnt is still mounted in the same place after getting namespace_sem, and it does take care to pin the dentry. It is needed, since otherwise we might end up with racing mount --move (or umount) happening while we were getting locks; in that case dentry would no longer be a mountpoint and could've been evicted on memory pressure along with its inode - not something you want when grabbing lock on that inode.

However, pinning a dentry is not enough - the matching mount is also pinned only by the fact that path->mnt is mounted on top it and at that point we are not holding any locks whatsoever, so the same kind of races could end up with all references to that mount gone just as we are about to enter inode_lock(). If that happens, we are left with filesystem being shut down while we are holding a dentry reference on it; results are not pretty.

What we need to do is grab both dentry and mount at the same time; that makes inodelock() safe *and* avoids the problem with fs getting shut down under us. After taking namespacesem we verify that path->mnt is still mounted (which stabilizes its ->mntparent) and check that it's still mounted at the same place. From that point on to the matching namespaceunlock() we are guaranteed that mount/dentry pair we'd grabbed are also pinned by being the mountpoint of path->mnt, so we can quietly drop both the dentry reference (as the current code does) and mnt one - it's OK to do under namespace_sem, since we are not dropping the final refs.

That solves the problem on dolockmount() side; unlockmount() also has one, since dentry is guaranteed to stay pinned only until the namespaceunlock(). That's easy to fix - just have inodeunlock() done earlier, while it's still pinned by mp->mdentry.