binfmt_misc: don't leak the user namespace when the mount fails

bm_get_tree() takes a reference to the user namespace and hands it to
get_tree_keyed() as the sget key. sget_fc() moves that reference into
sb->s_fs_info and clears fc->s_fs_info, so from that point on the
superblock owns it and bm_free() doesn't see it anymore.

The superblock drops it in ->put_super(). But generic_shutdown_super()
only calls ->put_super() from inside the if (sb->s_root) branch, so
nothing releases it when bm_fill_super() fails:

- The kzalloc_obj() failure leaves s_root NULL and the whole branch is
  skipped.

- A simple_fill_super() failure in the file loop leaves s_root set, but
  s_op still points at simple_super_operations, which has no
  ->put_super(). bm_fill_super() installs s_ops only once
  simple_fill_super() returned success, and installing it earlier
  wouldn't help either because simple_fill_super() overwrites s_op.

Either way vfs_get_super() calls deactivate_locked_super() and the
reference is gone for good. binfmt_misc mounts are available in a user
namespace and both the inode and the dentry cache are SLAB_ACCOUNT, so
an unprivileged caller under a tight memory cgroup can fail
simple_fill_super() on demand and leak one user namespace per attempt.

Drop the reference in ->kill_sb() instead, which runs unconditionally,
the same way nfsd and rpc_pipefs release their keyed s_fs_info.

That also stops ->put_super() from clearing s_fs_info while the
superblock is still on @fs_supers. generic_shutdown_super() leaves it
there on purpose so that sget_fc() keeps finding it until kill_sb() has
run, but a NULL s_fs_info makes test_keyed_super() miss it, so a
concurrent mount for the same user namespace skips the grab_super()
wait and creates a second superblock for a namespace that is still
being torn down.

Link: https://patch.msgid.link/20260728-work-binfmt_misc-usernsleak-v1-1-dbd8d5e626e7@kernel.org
Fixes: 21ca59b365 ("binfmt_misc: enable sandboxed mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/binfmt_misc.c

@@ -921,18 +921,9 @@ static const struct file_operations bm_status_operations = {
 
 /* Superblock handling */
 
-static void bm_put_super(struct super_block *sb)
-{
-	struct user_namespace *user_ns = sb->s_fs_info;
-
-	sb->s_fs_info = NULL;
-	put_user_ns(user_ns);
-}
-
 static const struct super_operations s_ops = {
 	.statfs		= simple_statfs,
 	.evict_inode	= bm_evict_inode,
-	.put_super	= bm_put_super,
 };
 
 static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
@@ -990,13 +981,12 @@ static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
 	/*
 	 * When the binfmt_misc superblock for this userns is shutdown
 	 * ->enabled might have been set to false and we don't reinitialize
-	 * ->enabled again in put_super() as someone might already be mounting
-	 * binfmt_misc again. It also would be pointless since by the time
-	 * ->put_super() is called we know that the binary type list for this
-	 * bintfmt_misc mount is empty making load_misc_binary() return
-	 * -ENOEXEC independent of whether ->enabled is true. Instead, if
-	 * someone mounts binfmt_misc for the first time or again we simply
-	 * reset ->enabled to true.
+	 * ->enabled again during shutdown as someone might already be mounting
+	 * binfmt_misc again. It also would be pointless since by then we know
+	 * that the binary type list for this binfmt_misc mount is empty making
+	 * load_misc_binary() return -ENOEXEC independent of whether ->enabled
+	 * is true. Instead, if someone mounts binfmt_misc for the first time or
+	 * again we simply reset ->enabled to true.
 	 */
 	misc->enabled = true;
 
@@ -1022,6 +1012,14 @@ static const struct fs_context_operations bm_context_ops = {
 	.get_tree	= bm_get_tree,
 };
 
+static void bm_kill_sb(struct super_block *sb)
+{
+	struct user_namespace *user_ns = sb->s_fs_info;
+
+	kill_anon_super(sb);
+	put_user_ns(user_ns);
+}
+
 static int bm_init_fs_context(struct fs_context *fc)
 {
 	fc->ops = &bm_context_ops;
@@ -1038,7 +1036,7 @@ static struct file_system_type bm_fs_type = {
 	.name		= "binfmt_misc",
 	.init_fs_context = bm_init_fs_context,
 	.fs_flags	= FS_USERNS_MOUNT,
-	.kill_sb	= kill_anon_super,
+	.kill_sb	= bm_kill_sb,
 };
 MODULE_ALIAS_FS("binfmt_misc");