On Fri, Oct 12, 2018 at 03:49:50PM +0100, Alan Jenkins wrote:

quoted

+SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags)
+{
+	struct fs_context *fc;
+	struct path target;
+	unsigned int lookup_flags;
+	int ret;
+
+	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
+		return -EPERM;

This seems to accept basically any mount.  Specifically: are you sure it's
OK to return a handle to a SB_NO_USER superblock?

Umm...  As long as we don't try to do pathname resolution from its ->s_root,
shouldn't be a problem and I don't see anything that would do that.  I might've
missed something, but...