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Date:   Sun, 4 Apr 2021 18:50:10 +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 07:05:13PM +0200, Christian Brauner wrote:
> On Sun, Apr 04, 2021 at 04:44:06PM +0000, Al Viro wrote:
> > 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?
> 
> Yeah, I have at least namei.o
> 
> https://drive.google.com/file/d/1AvO1St0YltIrA86DXjp1Xg3ojtS9owGh/view?usp=sharing

*grumble*

Is it reproducible without KASAN?  Would be much easier to follow the produced
asm...