[PATCH 04/17] namespace: check a recursive bind mount for mount namespace loops

[Christian Brauner <brauner@kernel.org>]

do_loopback() refuses to bind mount the file of a mount namespace that
is as old as the caller's or older because a mount namespace that holds
a mount of its own file, or of an ancestor's, would cause a cycle. But
it only checks the dentry the bind mount starts from. With MS_REC
everything below it is copied and do_loopback() passes
CL_COPY_MNT_NS_FILE so mount namespace files below the source are
copied.

That's fine for a source in the caller's own mount namespace. Every
mount namespace file in there passed the same check when it was
bind-mounted. It isn't fine for a source in another mount namespace.
may_copy_tree() accepts a bind mount of any nsfs or pidfs file no matter
what mount namespace it lives in so that /proc/<pid>/ns/<ns> and pidfds
can be bind mounted. If we create a bind-mount stack of mount namespaces
file descriptors the kernel will copy them irrespective of their
ancestoral relationship to the mount namespace in question:

  151 146 0:7 net:[4026531833] /tmp/nrc/y rw - nsfs nsfs rw
  152 151 0:7 mnt:[4026532293] /tmp/nrc/y rw - nsfs nsfs rw

Mount 152 is a mount of the mount namespace file of mount namespace
4026532293 inside mount namespace 4026532293. That namespace and every
mount in it are leaked... All it takes is a process in an older mount
namespace that stacks the file and repeating it leaks without limit:

  after control: Shmem:               380 kB
  child: mount(/proc/self/fd/6, MS_BIND|MS_REC): ok
  after cycle: Shmem:             65916 kB

do_move_mount() runs check_for_nsfs_mounts() over a detached tree for
exactly that reason. Do the same for the copy before it is grafted.

Fixes: e149ed2b805f ("take the targets of /proc/*/ns/* symlinks to separate fs")
Fixes: ef4144ac2dec ("pidfs: allow bind-mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/namespace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 23d3bfa9c14d..c2f54636ec9d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3055,7 +3055,11 @@ static int do_loopback(const struct path *path, const char *old_name,
 	if (IS_ERR(mnt))
 		return PTR_ERR(mnt);
 
-	err = graft_tree(mnt, &mp);
+	/* the copy may carry mount namespace files from below the source */
+	if (recurse && !check_for_nsfs_mounts(mnt))
+		err = -EINVAL;
+	else
+		err = graft_tree(mnt, &mp);
 	if (err) {
 		lock_mount_hash();
 		umount_tree(mnt, UMOUNT_SYNC);

-- 
2.53.0