[PATCH][RFC] do_lock_mount() races in 'beneath' case

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Apr 21, 2025 at 06:03:19PM +0100, Al Viro wrote:
> On Mon, Apr 21, 2025 at 05:29:47PM +0100, Al Viro wrote:
> 
> > What's to prevent the 'beneath' case from getting mnt mount --move'd
> > away *AND* the ex-parent from getting unmounted while we are blocked
> > in inode_lock?  At this point we are not holding any locks whatsoever
> > (and all mount-related locks nest inside inode_lock(), so we couldn't
> > hold them there anyway).
> > 
> > Hit that race and watch a very unhappy umount...
> 
> While we are at it, in normal case inode_unlock() in unlock_mount()
> is safe since we have dentry (and associated mount) pinned by
> struct path we'd fed to matching lock_mount().  No longer true for
> the 'beneath' case, AFAICS...

Completely untested patch follows; 'beneath' case in do_lock_mount() is made
to grab mount reference to match the dentry one (same lifetime; dropped
simultaneously), unlock_mount() unlocks the inode *before* namespace_unlock(),
so we don't depend upon the externally held references.

Comments?

diff --git a/fs/namespace.c b/fs/namespace.c
index fa17762268f5..91d28f283f1c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2827,56 +2827,62 @@ static struct mountpoint *do_lock_mount(struct path *path, bool beneath)
 	struct vfsmount *mnt = path->mnt;
 	struct dentry *dentry;
 	struct mountpoint *mp = ERR_PTR(-ENOENT);
+	struct path under = {};
 
 	for (;;) {
-		struct mount *m;
+		struct mount *m = real_mount(mnt);
 
 		if (beneath) {
-			m = real_mount(mnt);
+			path_put(&under);
 			read_seqlock_excl(&mount_lock);
-			dentry = dget(m->mnt_mountpoint);
+			under.mnt = mntget(&m->mnt_parent->mnt);
+			under.dentry = dget(m->mnt_mountpoint);
 			read_sequnlock_excl(&mount_lock);
+			dentry = under.dentry;
 		} else {
 			dentry = path->dentry;
 		}
 
 		inode_lock(dentry->d_inode);
-		if (unlikely(cant_mount(dentry))) {
-			inode_unlock(dentry->d_inode);
-			goto out;
-		}
-
 		namespace_lock();
 
-		if (beneath && (!is_mounted(mnt) || m->mnt_mountpoint != dentry)) {
+		if (unlikely(cant_mount(dentry) || !is_mounted(mnt)))
+			break;		// not to be mounted on
+
+		if (beneath && unlikely(m->mnt_mountpoint != dentry ||
+				        &m->mnt_parent->mnt != under.mnt)) {
 			namespace_unlock();
 			inode_unlock(dentry->d_inode);
-			goto out;
+			continue;	// got moved
 		}
 
 		mnt = lookup_mnt(path);
-		if (likely(!mnt))
+		if (unlikely(mnt)) {
+			namespace_unlock();
+			inode_unlock(dentry->d_inode);
+			path_put(path);
+			path->mnt = mnt;
+			path->dentry = dget(mnt->mnt_root);
+			continue;	// got overmounted
+		}
+		mp = get_mountpoint(dentry);
+		if (IS_ERR(mp))
 			break;
-
-		namespace_unlock();
-		inode_unlock(dentry->d_inode);
-		if (beneath)
-			dput(dentry);
-		path_put(path);
-		path->mnt = mnt;
-		path->dentry = dget(mnt->mnt_root);
-	}
-
-	mp = get_mountpoint(dentry);
-	if (IS_ERR(mp)) {
-		namespace_unlock();
-		inode_unlock(dentry->d_inode);
+		if (beneath) {
+			/*
+			 * @under duplicates the references that will stay
+			 * at least until namespace_unlock(), so the path_put()
+			 * below is safe (and OK to do under namespace_lock -
+			 * we are not dropping the final references here).
+			 */
+			path_put(&under);
+		}
+		return mp;
 	}
-
-out:
+	namespace_unlock();
+	inode_unlock(dentry->d_inode);
 	if (beneath)
-		dput(dentry);
-
+		path_put(&under);
 	return mp;
 }
 
@@ -2892,9 +2898,9 @@ static void unlock_mount(struct mountpoint *where)
 	read_seqlock_excl(&mount_lock);
 	put_mountpoint(where);
 	read_sequnlock_excl(&mount_lock);
+	inode_unlock(dentry->d_inode);
 
 	namespace_unlock();
-	inode_unlock(dentry->d_inode);
 }
 
 static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)