fs/fhandle.c: fix a race in call of has_locked_children()

BugLink: https://bugs.launchpad.net/bugs/2119603

[ Upstream commit 1f282cd ]

may_decode_fh() is calling has_locked_children() while holding no locks.
That's an oopsable race...

The rest of the callers are safe since they are holding namespace_sem and
are guaranteed a positive refcount on the mount in question.

Rename the current has_locked_children() to __has_locked_children(), make
it static and switch the fs/namespace.c users to it.

Make has_locked_children() a wrapper for __has_locked_children(), calling
the latter under read_seqlock_excl(&mount_lock).

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Fixes: 620c266 ("fhandle: relax open_by_handle_at() permission checks")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
CVE-2025-38306
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Mehmet Basaran <mehmet.basaran@canonical.com>

Author: Al Viro

fs/namespace.c

@@ -2271,7 +2271,7 @@ void drop_collected_mounts(struct vfsmount *mnt)
 	namespace_unlock();
 }
 
-bool has_locked_children(struct mount *mnt, struct dentry *dentry)
+static bool __has_locked_children(struct mount *mnt, struct dentry *dentry)
 {
 	struct mount *child;
 
@@ -2307,7 +2307,7 @@ struct vfsmount *clone_private_mount(const struct path *path)
 	if (!check_mnt(old_mnt))
 		goto invalid;
 
-	if (has_locked_children(old_mnt, path->dentry))
+	if (__has_locked_children(old_mnt, path->dentry))
 		goto invalid;
 
 	new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
@@ -2793,7 +2793,7 @@ static struct mount *__do_loopback(struct path *old_path, int recurse)
 			return mnt;
 	}
 
-	if (!recurse && has_locked_children(old, old_path->dentry))
+	if (!recurse && __has_locked_children(old, old_path->dentry))
 		return mnt;
 
 	if (recurse)
@@ -3128,6 +3128,16 @@ static inline int tree_contains_unbindable(struct mount *mnt)
 	return 0;
 }
 
+bool has_locked_children(struct mount *mnt, struct dentry *dentry)
+{
+	bool res;
+
+	read_seqlock_excl(&mount_lock);
+	res = __has_locked_children(mnt, dentry);
+	read_sequnlock_excl(&mount_lock);
+	return res;
+}
+
 /*
  * Check that there aren't references to earlier/same mount namespaces in the
  * specified subtree.  Such references can act as pins for mount namespaces
@@ -3190,7 +3200,7 @@ static int do_set_group(struct path *from_path, struct path *to_path)
 		goto out;
 
 	/* From mount should not have locked children in place of To's root */
-	if (has_locked_children(from, to->mnt.mnt_root))
+	if (__has_locked_children(from, to->mnt.mnt_root))
 		goto out;
 
 	/* Setting sharing groups is only allowed on private mounts */