[PATCH] vfs: move_mount: reject moving kernel internal mounts

[Eric Biggers <ebiggers@xxxxxxxxxx>]

From: Eric Biggers <ebiggers@xxxxxxxxxx>

sys_move_mount() crashes by dereferencing the pointer MNT_NS_INTERNAL,
a.k.a. ERR_PTR(-EINVAL), if the old mount is specified by fd for a
kernel object with an internal mount, such as a pipe or memfd.

Fix it by checking for this case and returning -EINVAL.

Reproducer:

    #include <unistd.h>

    #define __NR_move_mount         429
    #define MOVE_MOUNT_F_EMPTY_PATH 0x00000004

    int main()
    {
    	int fds[2];

    	pipe(fds);
        syscall(__NR_move_mount, fds[0], "", -1, "/", MOVE_MOUNT_F_EMPTY_PATH);
    }

Reported-by: syzbot+6004acbaa1893ad013f0@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 2db154b3ea8e ("vfs: syscall: Add move_mount(2) to move mounts around")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7660c2749c96..a7e5a44770a7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2600,7 +2600,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 	if (attached && !check_mnt(old))
 		goto out;
 
-	if (!attached && !(ns && is_anon_ns(ns)))
+	if (!attached && !(ns && ns != MNT_NS_INTERNAL && is_anon_ns(ns)))
 		goto out;
 
 	if (old->mnt.mnt_flags & MNT_LOCKED)
-- 
2.22.0