[PATCH 2/3] binfmt_misc: don't warn when the mount is completed from another user namespace

fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor. Nothing ties the task that calls
fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The
fd is inherited across fork() and exec() and it can be passed over a
unix socket.

Completing a context from another user namespace is allowed on purpose.
vfs_cmd_create() authorizes the create with mount_capable(), which for
FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that
succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns.
So an unprivileged task can reach the WARN_ON() in bm_fill_super():
create a user and a mount namespace in a child, call
fsopen("binfmt_misc") there, send the fscontext fd to the parent and let
the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain
unshare(1) and no capability is needed anywhere:

  WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]
  CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn
  Call Trace:
   get_tree_keyed+0x7d/0xb0
   bm_get_tree+0x34/0x90 [binfmt_misc]
   vfs_get_tree+0x2a/0x100
   vfs_cmd_create+0x60/0xf0
   __do_sys_fsconfig+0x4b2/0x500

The child needs the mount namespace because fsopen() itself gates on
may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning
the caller's mount namespace. fsconfig() doesn't repeat that check.

It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be
raised in a loop to taint the kernel and flood the log, and it panics a
kernel booted with panic_on_warn.

Keep refusing the mount and stop warning about it. Nothing in
bm_fill_super() depends on the two namespaces matching, it derives
everything from sb->s_user_ns.

Fixes: 21ca59b365c0 ("binfmt_misc: enable sandboxed mounts")
Cc: [email protected] # v6.7+
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 fs/binfmt_misc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index c97f10b48b5b..613dd28e3f1a 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -937,7 +937,8 @@ static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
 		/* last one */ {""}
 	};
 
-	if (WARN_ON(user_ns != current_user_ns()))
+	/* The fscontext fd may have been passed to another user namespace. */
+	if (user_ns != current_user_ns())
 		return -EINVAL;
 
 	/* Never exec off this instance and never let anything stack on it. */

-- 
2.53.0