Re: general protection fault in do_move_mount (2)

[Dmitry Vyukov]

On Mon, Jul 1, 2019 at 5:18 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> On Mon, Jul 01, 2019 at 04:59:04PM +0200, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> > >
> > > Dmitry, any idea why syzbot found such a bizarre reproducer for this?
> > > This is actually reproducible by a simple single threaded program:
> > >
> > >     #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);
> > >     }
> >
> >
> > There is no pipe in the reproducer, so it could not theoretically come
> > up with the reproducer with the pipe. During minimization syzkaller
> > only tries to remove syscalls and simplify arguments and execution
> > mode.
> > What would be the simplest reproducer expressed as further
> > minimization of this reproducer?
> > https://syzkaller.appspot.com/x/repro.syz?x=154e8c2aa00000
> > I assume one of the syscalls is still move_mount, but what is the
> > other one? If it's memfd_create, or open of the procfs file, then it
> > seems that [ab]used heavy threading and syscall colliding as way to do
> > an arbitrary mutation of the program. Per se results of
> > memfd_create/procfs are not passed to move_mount. But by abusing races
> > it probably managed to do so in small percent of cases. It would also
> > explain why it's hard to reproduce.
>
> To be clear, memfd_create() works just as well:
>
>         #define _GNU_SOURCE
>         #include <sys/mman.h>
>         #include <unistd.h>
>
>         #define __NR_move_mount         429
>         #define MOVE_MOUNT_F_EMPTY_PATH 0x00000004
>
>         int main()
>         {
>                 int fd = memfd_create("foo", 0);
>
>                 syscall(__NR_move_mount, fd, "", -1, "/", MOVE_MOUNT_F_EMPTY_PATH);
>         }
>
> I just changed it to pipe() in my example, because pipe() is less obscure.

Then I think the reason for the bizarre reproducer is what I described above.