Re: [syzbot] KMSAN: uninit-value in video_usercopy (2)

[Hans Verkuil]

On 16/03/2021 16:06, Arnd Bergmann wrote:
> On Tue, Mar 16, 2021 at 3:02 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> On Tue, Mar 16, 2021 at 2:56 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>> On Tue, Mar 16, 2021 at 11:44 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>>> On Tue, Mar 16, 2021 at 11:18 AM syzbot
>>>> <syzbot+142888ffec98ab194028@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> Hello,
>>>>>
>>>>> syzbot found the following issue on:
>>>>>
>>>>> HEAD commit:    29ad81a1 arch/x86: add missing include to sparsemem.h
>>>
>>> This tree seems to be missing fb18802a338b ("media: v4l: ioctl: Fix memory
>>> leak in video_usercopy"), which rewrote that function partly and might
>>> fix the problem.
>>>
>>>>> Local variable ----sbuf@video_usercopy created at:
>>>>>  video_usercopy+0xaa/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3285
>>>>>  video_usercopy+0xaa/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3285
>>>>> =====================================================
>>>>> =====================================================
>>>>> BUG: KMSAN: uninit-value in check_fmt+0x864/0x1070 drivers/media/v4l2-core/v4l2-ioctl.c:963
>>>>> CPU: 0 PID: 19595 Comm: syz-executor.4 Tainted: G    B             5.11.0-rc7-syzkaller #0
>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>>>>> Call Trace:
>>>>>  __dump_stack lib/dump_stack.c:79 [inline]
>>>>>  dump_stack+0x21c/0x280 lib/dump_stack.c:120
>>>>>  kmsan_report+0xfb/0x1e0 mm/kmsan/kmsan_report.c:118
>>>>>  __msan_warning+0x5f/0xa0 mm/kmsan/kmsan_instr.c:197
>>>>>  check_fmt+0x864/0x1070 drivers/media/v4l2-core/v4l2-ioctl.c:963
>>>>>  v4l_prepare_buf+0xbf/0x1d0 drivers/media/v4l2-core/v4l2-ioctl.c:2107
>>>>>  __video_do_ioctl+0x15cd/0x1d20 drivers/media/v4l2-core/v4l2-ioctl.c:2993
>>>>>  video_usercopy+0x2313/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3345
>>>>>  video_ioctl2+0x9f/0xb0 drivers/media/v4l2-core/v4l2-ioctl.c:3391
>>>>>  v4l2_ioctl+0x255/0x290 drivers/media/v4l2-core/v4l2-dev.c:360
>>>>>  v4l2_compat_ioctl32+0x2c6/0x370 drivers/media/v4l2-core/v4l2-compat-ioctl32.c:1248
>>>>>  __do_compat_sys_ioctl fs/ioctl.c:842 [inline]
>>>>>  __se_compat_sys_ioctl+0x53d/0x1100 fs/ioctl.c:793
>>>>>  __ia32_compat_sys_ioctl+0x4a/0x70 fs/ioctl.c:793
>>>>>  do_syscall_32_irqs_on arch/x86/entry/common.c:79 [inline]
>>>>>  __do_fast_syscall_32+0x102/0x160 arch/x86/entry/common.c:141
>>>>>  do_fast_syscall_32+0x6a/0xc0 arch/x86/entry/common.c:166
>>>>>  do_SYSENTER_32+0x73/0x90 arch/x86/entry/common.c:209
>>>>>  entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
>>>>> RIP: 0023:0xf7fec549
>>>>> Code: 03 74 c0 01 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
>>>>> RSP: 002b:00000000f55e65fc EFLAGS: 00000296 ORIG_RAX: 0000000000000036
>>>>> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000c050565d
>>>>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
>>>>> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>>>>> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>>>>> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>>>>>
>>>>> Local variable ----sbuf@video_usercopy created at:
>>>>>  video_usercopy+0xaa/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3285
>>>>>  video_usercopy+0xaa/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3285
>>>>> =====================================================
>>>>
>>>> I did not get to the very bottom of this, but I looked at this a bit.
>>>> It seems to be related to some unfortunate interaction of compat
>>>> syscall and CONFIG_COMPAT_32BIT_TIME. It seems that in this case
>>>> nothing at all is copied from userspace because cmd gets messed up or
>>>> something. Perhaps VIDIOC_QUERYBUF is translated into
>>>> VIDIOC_QUERYBUF_TIME32 instead of VIDIOC_QUERYBUF32_TIME32 and then
>>>> this gets into compat syscall path and v4l2_compat_get_user does not
>>>> recognize the command, copies nothing but returns 0.
>>>
>>> User space would be calling VIDIOC_QUERYBUF32_TIME32 here,
>>> if it's built against glibc, though with a musl based user space, you
>>> would get called with VIDIOC_QUERYBUF32.
>>
>> Or somebody fetching somebody else's credit card number will be
>> calling VIDIOC_QUERYBUF_TIME32 directly ;)
> 
> Ah of course, I forgot the ioctl command may already be fuzzed here.
> 
> When I look at
> https://syzkaller.appspot.com/text?tag=CrashLog&x=12bd0e3ad00000
> 
> I see 0xc0585609, which would be a VIDIOC_QUERYBUF with
> size=0x58, which is the native ioctl, not the compat one. This
> is something we didn't expect to get passed into the compat ioctl
> handler, but should of course handle gracefully
> 
> If the command were to get is the 64-bit version of
> VIDIOC_QUERYBUF_TIME32 (0xc0505609), then it gets converted to
> VIDIOC_QUERYBUF by video_translate_cmd().
> If it's VIDIOC_QUERYBUF, it stays that way.
> 
> It does break down in v4l2_compat_get_user() when we get
> called with VIDIOC_QUERYBUF_TIME32, since that leads
> to not copying at all, as you guessed.
> 
> I think this should fix the case of passing
> VIDIOC_QUERYBUF_TIME32:

I tested this and I can confirm that this works.

Arnd, do you want to make a patch for this? If so, you can add my

Tested-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>

Regards,

	Hans

> 
> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> @@ -3115,7 +3115,7 @@ static int check_array_args(unsigned int cmd,
> void *parg, size_t *array_size,
>  static unsigned int video_translate_cmd(unsigned int cmd)
>  {
>         switch (cmd) {
> -#ifdef CONFIG_COMPAT_32BIT_TIME
> +#if !defined(CONFIG_64BIT) && defined(CONFIG_COMPAT_32BIT_TIME)
>         case VIDIOC_DQEVENT_TIME32:
>                 return VIDIOC_DQEVENT;
>         case VIDIOC_QUERYBUF_TIME32:
> 
>        Arnd
>