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

From: Al Viro <viro@zeniv.linux.org.uk>
To: Eric Biggers <ebiggers@kernel.org>
Cc: David Howells <dhowells@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] vfs: move_mount: reject moving kernel internal mounts
Date: Mon, 1 Jul 2019 19:22:39 +0100	[thread overview]
Message-ID: <20190701182239.GA17978@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20190701164536.GA202431@gmail.com>

On Mon, Jul 01, 2019 at 09:45:37AM -0700, Eric Biggers wrote:
> On Sat, Jun 29, 2019 at 01:27:44PM -0700, Eric Biggers wrote:
> > 
> > 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);
> >     }
> 
> David, I'd like to add this as a regression test somewhere.
> 
> Can you point me to the tests for the new mount syscalls?
> 
> I checked LTP, kselftests, and xfstests, but nothing to be found.

FWIW, it's not just move_mount(2) - I'd expect

	int fds[2];
	char s[80];

	pipe(fds);
	sprintf(s, "/dev/fd/%d", fds[0]);
	mount(s, "/dev/null", NULL, MS_MOVE, 0);

to step into exactly the same thing.  mount(2) does follow symlinks -
always had...