Re: KMSAN: uninit-value in snd_midi_event_encode_byte

[Takashi Iwai]

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...


thanks,

Takashi

> 
> 
> 
> > Takashi
> 
> > -- 8< --
> > From: Takashi Iwai <tiwai@xxxxxxx>
> > Subject: [PATCH] ALSA: rawmidi: Initialize allocated buffers
> 
> > syzbot reported the uninitialized value exposure in certain situations
> > using virmidi loop.  It's likely a very small race at writing and
> > reading, and the influence is almost negligible.  But it's safer to
> > paper over this just by replacing the existing kvmalloc() with
> > kvzalloc().
> 
> > Reported-by: syzbot+194dffdb8b22fc5d207a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > ---
> >   sound/core/rawmidi.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> > diff --git a/sound/core/rawmidi.c b/sound/core/rawmidi.c
> > index 69517e18ef07..08d5662039e3 100644
> > --- a/sound/core/rawmidi.c
> > +++ b/sound/core/rawmidi.c
> > @@ -129,7 +129,7 @@ static int snd_rawmidi_runtime_create(struct
> > snd_rawmidi_substream *substream)
> >   		runtime->avail = 0;
> >   	else
> >   		runtime->avail = runtime->buffer_size;
> > -	runtime->buffer = kvmalloc(runtime->buffer_size, GFP_KERNEL);
> > +	runtime->buffer = kvzalloc(runtime->buffer_size, GFP_KERNEL);
> >   	if (!runtime->buffer) {
> >   		kfree(runtime);
> >   		return -ENOMEM;
> > @@ -655,7 +655,7 @@ static int resize_runtime_buffer(struct
> > snd_rawmidi_runtime *runtime,
> >   	if (params->avail_min < 1 || params->avail_min > params->buffer_size)
> >   		return -EINVAL;
> >   	if (params->buffer_size != runtime->buffer_size) {
> > -		newbuf = kvmalloc(params->buffer_size, GFP_KERNEL);
> > +		newbuf = kvzalloc(params->buffer_size, GFP_KERNEL);
> >   		if (!newbuf)
> >   			return -ENOMEM;
> >   		spin_lock_irq(&runtime->lock);
> > --
> > 2.18.0
>