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Date:   Sun, 4 Apr 2021 16:44:06 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com>,
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Subject: Re: [syzbot] WARNING in mntput_no_expire (2)
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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?