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From: Dmitry Vyukov <dvyukov@google.com>
To: Eric Biggers <ebiggers@kernel.org>
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: Fri, 5 Jul 2019 14:17:04 +0200	[thread overview]
Message-ID: <CACT4Y+YxjcY2wMJm2THYi6KouQ1dzyGTNEjOe8wSqj8in2qigw@mail.gmail.com> (raw)
In-Reply-To: <20190701151808.GA790@sol.localdomain>

On Mon, Jul 1, 2019 at 5:18 PM Eric Biggers <ebiggers@kernel.org> 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.

  reply	other threads:[~2019-07-05 12:17 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-18 10:47 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
2019-07-05 12:17           ` Dmitry Vyukov [this message]
2019-07-05 13:02           ` Dmitry Vyukov

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