From: Eric Biggers <ebiggers@kernel.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
syzbot <syzbot+6004acbaa1893ad013f0@syzkaller.appspotmail.com>,
Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@kernel.dk>,
Borislav Petkov <bp@alien8.de>,
Catalin Marinas <catalin.marinas@arm.com>,
Christian Brauner <christian@brauner.io>,
David Howells <dhowells@redhat.com>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Hannes Reinecke <hare@suse.com>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
"H. Peter Anvin" <hpa@zytor.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
Thomas Gleixner <tglx@linutronix.de>,
the arch/x86 maintainers <x86@kernel.org>
Subject: Re: general protection fault in do_move_mount (2)
Date: Mon, 1 Jul 2019 08:18:08 -0700 [thread overview]
Message-ID: <20190701151808.GA790@sol.localdomain> (raw)
In-Reply-To: <CACT4Y+aAqEyJdjTzRksGuFmnTjDHbB9yS6bPsK52sz3+jhxNbw@mail.gmail.com>
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.
>
>
> > FYI, it also isn't really appropriate for syzbot to bisect all bugs in new
> > syscalls to wiring them up to x86, and then blame all the x86 maintainers.
> > Normally such bugs will be in the syscall itself, regardless of architecture.
>
> Agree. Do you think it's something worth handling automatically
> (stands out of the long tail of other inappropriate cases)? If so, how
> could we detect such cases? It seems that some of these predicates are
> quite hard to program. Similar things happen with introduction of new
> bug detection tools and checks, wiring any functionality to new access
> points and similar things.
>
Yes, this case could easily be automatically detected (most of the time) by
listing the filenames changed in the commit, and checking whether they all match
the pattern syscall.*\.tbl. Sure, it's not common, but it could be alongside
other similar straightforward checks like checking for merge commits and
checking for commits that only modify Documentation/.
I'm not even asking for more correct bisection results at this point, I'm just
asking for fewer bad bisection results.
- Eric
next prev parent reply other threads:[~2019-07-01 15:18 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-18 10:47 general protection fault in do_move_mount (2) syzbot
2019-06-18 14:02 ` Al Viro
2019-06-24 9:28 ` Dmitry Vyukov
2019-06-29 20:27 ` [PATCH] vfs: move_mount: reject moving kernel internal mounts Eric Biggers
2019-06-29 20:39 ` Al Viro
2019-07-01 1:08 ` Al Viro
2019-07-01 15:43 ` Eric Biggers
2019-07-01 7:38 ` David Howells
2019-07-01 11:19 ` Al Viro
2019-07-01 16:45 ` Eric Biggers
2019-07-01 18:22 ` Al Viro
2019-07-01 19:20 ` Al Viro
2019-07-02 18:22 ` Eric Biggers
2019-07-09 19:40 ` Eric Biggers
2019-07-09 20:54 ` Al Viro
2019-07-10 3:23 ` 6 new syscalls without tests (was: [PATCH] vfs: move_mount: reject moving kernel internal mounts) Eric Biggers
2019-07-05 9:01 ` move_mount.2 David Howells
2019-06-29 20:39 ` general protection fault in do_move_mount (2) Eric Biggers
2019-07-01 14:59 ` Dmitry Vyukov
2019-07-01 15:18 ` Eric Biggers [this message]
2019-07-05 12:17 ` Dmitry Vyukov
2019-07-05 13:02 ` Dmitry Vyukov
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