KASAN: user-memory-access Write in video_usercopy

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    26467385 Add linux-next specific files for 20200107
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10d44076e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2265a716722be976
dashboard link: https://syzkaller.appspot.com/bug?extid=54fd8cca4b7226c94b8e
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+54fd8cca4b7226c94b8e@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: user-memory-access in memset include/linux/string.h:411 [inline]
BUG: KASAN: user-memory-access in video_get_user+0x67f/0x890  
drivers/media/v4l2-core/v4l2-ioctl.c:3053
Write of size 512 at addr 00000000200001c0 by task syz-executor.2/1523

CPU: 1 PID: 1523 Comm: syz-executor.2 Not tainted  
5.5.0-rc5-next-20200107-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x197/0x210 lib/dump_stack.c:118
 __kasan_report.cold+0x5/0x32 mm/kasan/report.c:510
 kasan_report+0x12/0x20 mm/kasan/common.c:641
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
 memset+0x24/0x40 mm/kasan/common.c:108
 memset include/linux/string.h:411 [inline]
 video_get_user+0x67f/0x890 drivers/media/v4l2-core/v4l2-ioctl.c:3053
 video_usercopy+0x21f/0x10b0 drivers/media/v4l2-core/v4l2-ioctl.c:3210
 video_ioctl2+0x2d/0x35 drivers/media/v4l2-core/v4l2-ioctl.c:3274
 v4l2_ioctl+0x1ac/0x230 drivers/media/v4l2-core/v4l2-dev.c:360
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x123/0x180 fs/ioctl.c:747
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45af49
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f5917b07c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045af49
RDX: 00000000200001c0 RSI: 0000001002008914 RDI: 0000000000000006
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f5917b086d4
R13: 00000000004c2837 R14: 00000000004d8b30 R15: 00000000ffffffff
==================================================================


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