Re: [syzbot] WARNING in mntput_no_expire (2)

Al Viro
2021-04-04T16:44:11Z

On Sun, Apr 04, 2021 at 06:40:40PM +0200, Christian Brauner wrote:

> > Very interesting. What happens if you call loop() twice? And now I wonder
> > whether it's root or cwd, actually... Hmm...
> >
> > How about this:
> > fd = open("/proc/self/mountinfo", 0);
> > mkdir("./newroot/foo", 0777);
> > mount("./newroot/foo", "./newroot/foo", 0, MS_BIND, NULL);
> > chroot("./newroot");
> > chdir("/foo");
> > while (1) {
> > static char buf[4096];
> > int n = read(fd, buf, 4096);
> > if (n <= 0)
> > break;
> > write(1, buf, n);
> > }
> > close(fd);
> > drop_caps();
> > loop();
> > as the end of namespace_sandbox_proc(), instead of
> > chroot("./newroot");
> > chdir("/");
> > drop_caps();
> > loop();
> > sequence we have there?
>
> Uhum, well then we oops properly with a null-deref.

Cute... Could you dump namei.o (ideally - with namei.s) from your build
someplace public?