Re: [syzbot] kernel BUG in mcopy_continue

[Dmitry Vyukov]

On Mon, 13 Jun 2022 at 19:50, 'Axel Rasmussen' via syzkaller-bugs
<syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
>
> I think this is the problem Peter Xu pointed out, and has been fixed
> in v2 of the patch (which has us return -EFAULT instead of -ENOENT).
>
> I think this based on:
>
> This page has a link to the line on which the bug happened:
> https://syzkaller.appspot.com/bug?extid=3f1e5c1dea3a2e66ea9a
> It links to this line:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/mm/userfaultfd.c?id=6d0c806803170f120f8cb97b321de7bd89d3a791#n642
>
> So, it seems this is the BUG that was triggered:
>
> if (unlikely(err == -ENOENT)) {
>         BUG_ON(!page);
> }

Let's tell the bot this is fixed so that it reports such bugs in future:

#syz fix: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages



> On Mon, Jun 13, 2022 at 10:41 AM Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Sun, 12 Jun 2022 23:47:26 -0700 syzbot <syzbot+3f1e5c1dea3a2e66ea9a@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    6d0c80680317 Add linux-next specific files for 20220610
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=13a777f7f00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=a30d6e3e814e5931
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=3f1e5c1dea3a2e66ea9a
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+3f1e5c1dea3a2e66ea9a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > ------------[ cut here ]------------
> > > kernel BUG at mm/userfaultfd.c:642!
> >
> > Thanks.  I'm looking suspiciously at "mm: userfaultfd: fix
> > UFFDIO_CONTINUE on fallocated shmem pages"?
> >
> > > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > > CPU: 0 PID: 14685 Comm: syz-executor.4 Not tainted 5.19.0-rc1-next-20220610-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > RIP: 0010:__mcopy_atomic mm/userfaultfd.c:642 [inline]
> > > RIP: 0010:mcopy_continue+0x80f/0x1360 mm/userfaultfd.c:699
> > > Code: a8 ff 49 83 fd fe 0f 85 bd fd ff ff e8 ba ca a8 ff 0f 1f 44 00 00 e8 b0 ca a8 ff e8 ab ca a8 ff 48 8b 7c 24 20 e8 51 76 8b ff <0f> 0b e8 9a ca a8 ff 0f 0b e8 93 ca a8 ff 4d 89 ee 31 ff 41 81 e6
> > > RSP: 0018:ffffc900058f7b70 EFLAGS: 00010282
> > > RAX: 0000000000000000 RBX: ffff8880226dee58 RCX: 0000000000000000
> > > RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff88801a0b4228
> > > RBP: 0000000020531000 R08: 0000000000000001 R09: ffff88801a0b422f
> > > R10: ffffed1003416845 R11: 0000000000000001 R12: 0000000000000000
> > > R13: fffffffffffffffe R14: 00000000fffffffe R15: ffff88801a0b4140
> > > FS:  00007f91409ee700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000001b2c522000 CR3: 000000001d578000 CR4: 00000000003526f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > Call Trace:
> > >  <TASK>
> > >  userfaultfd_continue fs/userfaultfd.c:1893 [inline]
> > >  userfaultfd_ioctl+0x1818/0x41f0 fs/userfaultfd.c:2017
> > >  vfs_ioctl fs/ioctl.c:51 [inline]
> > >  __do_sys_ioctl fs/ioctl.c:870 [inline]
> > >  __se_sys_ioctl fs/ioctl.c:856 [inline]
> > >  __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> > > RIP: 0033:0x7f913f889109
> > > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > > RSP: 002b:00007f91409ee168 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > > RAX: ffffffffffffffda RBX: 00007f913f99c030 RCX: 00007f913f889109
> > > RDX: 0000000020000040 RSI: 00000000c020aa07 RDI: 0000000000000004
> > > RBP: 00007f913f8e30ad R08: 0000000000000000 R09: 0000000000000000
> > > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > > R13: 00007ffcb29c401f R14: 00007f91409ee300 R15: 0000000000022000
> > >  </TASK>
> > > Modules linked in:
> > > ---[ end trace 0000000000000000 ]---
> > > RIP: 0010:__mcopy_atomic mm/userfaultfd.c:642 [inline]
> > > RIP: 0010:mcopy_continue+0x80f/0x1360 mm/userfaultfd.c:699
> > > Code: a8 ff 49 83 fd fe 0f 85 bd fd ff ff e8 ba ca a8 ff 0f 1f 44 00 00 e8 b0 ca a8 ff e8 ab ca a8 ff 48 8b 7c 24 20 e8 51 76 8b ff <0f> 0b e8 9a ca a8 ff 0f 0b e8 93 ca a8 ff 4d 89 ee 31 ff 41 81 e6
> > > RSP: 0018:ffffc900058f7b70 EFLAGS: 00010282
> > > RAX: 0000000000000000 RBX: ffff8880226dee58 RCX: 0000000000000000
> > > RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffff88801a0b4228
> > > RBP: 0000000020531000 R08: 0000000000000001 R09: ffff88801a0b422f
> > > R10: ffffed1003416845 R11: 0000000000000001 R12: 0000000000000000
> > > R13: fffffffffffffffe R14: 00000000fffffffe R15: ffff88801a0b4140
> > > FS:  00007f91409ee700(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 00007f91409cd718 CR3: 000000001d578000 CR4: 00000000003526f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > >
> > >
> > > ---
> > > This report is generated by a bot. It may contain errors.
> > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
> > >
> > > syzbot will keep track of this issue. See:
> > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
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