WARNING: bad usercopy in put_cmsg_compat

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
e237f98a9c134c3d600353f21e07db915516875b (Mon Feb 5 21:35:56 2018 +0000)
Merge tag 'xfs-4.16-merge-5' of
git://git.kernel.org/pub/scm/fs/xfs/xfs-linux

C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.
user-space arch: i386

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Reported-by: syzbot+733048...@syzkaller.appspotmail.com
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------------[ cut here ]------------
Bad or missing usercopy whitelist? Kernel memory exposure attempt detected
from SLAB object 'skbuff_head_cache' (offset 64, size 16)!
WARNING: CPU: 0 PID: 3994 at mm/usercopy.c:81 usercopy_warn+0xdb/0x100
mm/usercopy.c:76
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 3994 Comm: syzkaller552561 Not tainted 4.15.0+ #210
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x257 lib/dump_stack.c:53
panic+0x1e4/0x41c kernel/panic.c:183
__warn+0x1dc/0x200 kernel/panic.c:547
report_bug+0x211/0x2d0 lib/bug.c:184
fixup_bug.part.11+0x37/0x80 arch/x86/kernel/traps.c:178
fixup_bug arch/x86/kernel/traps.c:247 [inline]
do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
invalid_op+0x22/0x40 arch/x86/entry/entry_64.S:984
RIP: 0010:usercopy_warn+0xdb/0x100 mm/usercopy.c:76
RSP: 0018:ffff8801cf3873f0 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: ffffffff86801907 RCX: ffffffff815a585e
RDX: 0000000000000000 RSI: 1ffff10039e70e2e RDI: 1ffff10039e70e03
RBP: ffff8801cf387448 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000866 R11: 0000000000000000 R12: ffffffff86403180
R13: ffffffff85f2d4c0 R14: 0000000000000040 R15: 0000000000000010
__check_heap_object+0x89/0xc0 mm/slab.c:4426
check_heap_object mm/usercopy.c:236 [inline]
__check_object_size+0x272/0x530 mm/usercopy.c:259
check_object_size include/linux/thread_info.h:112 [inline]
check_copy_size include/linux/thread_info.h:143 [inline]
copy_to_user include/linux/uaccess.h:154 [inline]
put_cmsg_compat+0x724/0xa50 net/compat.c:254
put_cmsg+0x33a/0x3f0 net/core/scm.c:225
sock_recv_errqueue+0x200/0x3e0 net/core/sock.c:2910
packet_recvmsg+0xb2e/0x17a0 net/packet/af_packet.c:3296
sock_recvmsg_nosec net/socket.c:803 [inline]
sock_recvmsg+0xc9/0x110 net/socket.c:810
___sys_recvmsg+0x2a4/0x640 net/socket.c:2205
__sys_recvmsg+0xe2/0x210 net/socket.c:2250
C_SYSC_recvmsg net/compat.c:751 [inline]
compat_SyS_recvmsg+0x2a/0x40 net/compat.c:749
do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
do_fast_syscall_32+0x3ee/0xfa1 arch/x86/entry/common.c:392
entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129
RIP: 0023:0xf7f45c79
RSP: 002b:00000000fffebddc EFLAGS: 00000217 ORIG_RAX: 0000000000000174
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000020006fc8
RDX: 0000000000002000 RSI: 0000000000000004 RDI: 0000000000000004
RBP: 0000000020000fe6 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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[Kees Cook]

#syz dup: WARNING in usercopy_warn

Same root cause and will have the same fix.

-Kees

--
Kees Cook
Pixel Security