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

[Dmitry Vyukov]

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
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=102502dcd00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8b976581f6bd1e7d
> dashboard link: https://syzkaller.appspot.com/bug?extid=142888ffec98ab194028
> compiler:       Debian clang version 11.0.1-2
> userspace arch: i386
>
> 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+142888ffec98ab194028@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> =====================================================
> BUG: KMSAN: uninit-value in check_array_args drivers/media/v4l2-core/v4l2-ioctl.c:3041 [inline]
> BUG: KMSAN: uninit-value in video_usercopy+0x1631/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3315
> CPU: 0 PID: 19595 Comm: syz-executor.4 Not tainted 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_array_args drivers/media/v4l2-core/v4l2-ioctl.c:3041 [inline]
>  video_usercopy+0x1631/0x3d30 drivers/media/v4l2-core/v4l2-ioctl.c:3315
>  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
> =====================================================
> =====================================================
> 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.


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