Re: KMSAN: uninit-value in snd_midi_event_encode_byte

From: Dmitry Vyukov
Date: Mon Sep 03 2018 - 09:42:03 EST

On Mon, Sep 3, 2018 at 3:37 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> On Mon, 03 Sep 2018 15:33:12 +0200,
> Dmitry Vyukov wrote:
>>
>> On Mon, Sep 3, 2018 at 3:29 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
>> > On Mon, 03 Sep 2018 15:22:42 +0200,
>> > syzbot wrote:
>> >>
>> >> > On Mon, 03 Sep 2018 03:53:03 +0200,
>> >> > syzbot wrote:
>> >>
>> >> >> syzbot has found a reproducer for the following crash on:
>> >>
>> >> >> HEAD commit: 28f0ca98eadf kmsan: don't instrument
>> >> >> do_syscall_64() and _..
>> >> >> git tree: https://github.com/google/kmsan.git/master
>> >> >> console output: https://syzkaller.appspot.com/x/log.txt?x=10556c92400000
>> >> >> kernel config:
>> >> >> https://syzkaller.appspot.com/x/.config?x=3431f03869413153
>> >> >> dashboard link:
>> >> >> https://syzkaller.appspot.com/bug?extid=194dffdb8b22fc5d207a
>> >> >> compiler: clang version 8.0.0 (trunk 339414)
>> >> >> syz repro:
>> >> >> https://syzkaller.appspot.com/x/repro.syz?x=123a520a400000
>> >> >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16068dc1400000
>> >>
>> >> >> IMPORTANT: if you fix the bug, please add the following tag to the
>> >> >> commit:
>> >> >> Reported-by: syzbot+194dffdb8b22fc5d207a@xxxxxxxxxxxxxxxxxxxxxxxxx
>> >>
>> >> >> ==================================================================
>> >> >> BUG: KMSAN: uninit-value in snd_midi_event_encode_byte+0x569/0xff0
>> >> >> sound/core/seq/seq_midi_event.c:195
>> >> >> CPU: 1 PID: 1659 Comm: kworker/1:1H Not tainted 4.19.0-rc1+ #40
>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine,
>> >> >> BIOS Google 01/01/2011
>> >> >> Workqueue: events_highpri snd_vmidi_output_work
>> >> >> Call Trace:
>> >> >> __dump_stack lib/dump_stack.c:77 [inline]
>> >> >> dump_stack+0x14b/0x190 lib/dump_stack.c:113
>> >> >> kmsan_report+0x183/0x2b0 mm/kmsan/kmsan.c:956
>> >> >> __msan_warning+0x70/0xc0 mm/kmsan/kmsan_instr.c:645
>> >> >> snd_midi_event_encode_byte+0x569/0xff0
>> >> >> sound/core/seq/seq_midi_event.c:195
>> >> >> snd_vmidi_output_work+0x34e/0x5b0 sound/core/seq/seq_virmidi.c:161
>> >> >> process_one_work+0x1605/0x1f40 kernel/workqueue.c:2153
>> >> >> worker_thread+0x11a2/0x2590 kernel/workqueue.c:2296
>> >> >> kthread+0x465/0x4a0 kernel/kthread.c:247
>> >> >> ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416
>> >>
>> >> >> Uninit was stored to memory at:
>> >> >> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:256 [inline]
>> >> >> kmsan_save_stack mm/kmsan/kmsan.c:271 [inline]
>> >> >> kmsan_internal_chain_origin+0x128/0x210 mm/kmsan/kmsan.c:573
>> >> >> __msan_chain_origin+0x69/0xc0 mm/kmsan/kmsan_instr.c:482
>> >> >> __snd_rawmidi_transmit_peek sound/core/rawmidi.c:1103 [inline]
>> >> >> snd_rawmidi_transmit+0xa75/0xbf0 sound/core/rawmidi.c:1228
>> >> >> snd_vmidi_output_work+0x2ac/0x5b0 sound/core/seq/seq_virmidi.c:159
>> >> >> process_one_work+0x1605/0x1f40 kernel/workqueue.c:2153
>> >> >> worker_thread+0x11a2/0x2590 kernel/workqueue.c:2296
>> >> >> kthread+0x465/0x4a0 kernel/kthread.c:247
>> >> >> ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416
>> >>
>> >> >> Uninit was created at:
>> >> >> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:256 [inline]
>> >> >> kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:181
>> >> >> kmsan_kmalloc+0x98/0x100 mm/kmsan/kmsan_hooks.c:91
>> >> >> __kmalloc_node+0x7bf/0x11c0 mm/slub.c:3828
>> >> >> kmalloc_node include/linux/slab.h:555 [inline]
>> >> >> kvmalloc_node+0x19d/0x3e0 mm/util.c:423
>> >> >> kvmalloc include/linux/mm.h:577 [inline]
>> >> >> snd_rawmidi_runtime_create sound/core/rawmidi.c:132 [inline]
>> >> >> open_substream+0x3c8/0xaa0 sound/core/rawmidi.c:276
>> >> >> rawmidi_open_priv+0x347/0x1000 sound/core/rawmidi.c:327
>> >> >> snd_rawmidi_open+0x7d4/0x1120 sound/core/rawmidi.c:424
>> >> >> soundcore_open+0x9be/0xa60 sound/sound_core.c:597
>> >> >> chrdev_open+0xc26/0xdb0 fs/char_dev.c:417
>> >> >> do_dentry_open+0xce6/0x1740 fs/open.c:771
>> >> >> vfs_open+0xaf/0xe0 fs/open.c:880
>> >> >> do_last fs/namei.c:3418 [inline]
>> >> >> path_openat+0x1799/0x6870 fs/namei.c:3534
>> >> >> do_filp_open+0x259/0x610 fs/namei.c:3564
>> >> >> do_sys_open+0x630/0x940 fs/open.c:1063
>> >> >> __do_sys_open fs/open.c:1081 [inline]
>> >> >> __se_sys_open+0xad/0xc0 fs/open.c:1076
>> >> >> __x64_sys_open+0x4a/0x70 fs/open.c:1076
>> >> >> do_syscall_64+0xb8/0x100 arch/x86/entry/common.c:291
>> >> >> entry_SYSCALL_64_after_hwframe+0x63/0xe7
>> >> >> ==================================================================
>> >>
>> >> > This looks like some small race at virmidi reading and the buffer
>> >> > handling, which should be almost harmless by itself.
>> >> > Nevertheless, the fix should be easy, just replacing kvmalloc() with
>> >> > kvzalloc(), as below.
>> >>
>> >> > Let's check whether this works.
>> >>
>> >> > #syz test:
>> >> > git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git
>> >> > topic/rawmidi-fixes
>> >>
>> >> KMSAN bugs can only be tested on https://github.com/google/kmsan.git tree
>> >> because KMSAN tool is not upstreamed yet.
>> >> See
>> >> https://github.com/google/syzkaller/blob/master/docs/syzbot.md#kmsan-bugs
>> >> for details.
>> >
>> > So how is the procedure to test a patch for bugs spotted by KMSAN?
>> >
>> > In this case, the fix is trivial, so I can put it to my tree for the
>> > next pull request. Of course, it'd be better if it can be confirmed
>> > to work beforehand...
>>
>> https://github.com/google/syzkaller/blob/master/docs/syzbot.md#kmsan-bugs
>>
>> Which means you need to attach a patch because it's not in that tree.
>
> OK, it would have been helpful if the description there was more clear
> :)
>
> And no automated way for that?

What exactly do you mean? Please elaborate.

> I mean, I already attached patch in
> the previous reply (together with syz-magic command to point the git
> branch URL). Do I need anything else?

Just issue the command against "https://github.com/google/kmsan.git
master" and attach the patch.