cxl/mbox: validate payload size before accessing contents in cxl_payload_from_user_allowed()

In the Linux kernel, the following vulnerability has been resolved:

cxl/mbox: validate payload size before accessing contents in cxl_payload_from_user_allowed()

cxl_payload_from_user_allowed() casts and dereferences the input
payload without first verifying its size. When a raw mailbox command
is sent with an undersized payload (ie: 1 byte for CXL_MBOX_OP_CLEAR_LOG,
which expects a 16-byte UUID), uuid_equal() reads past the allocated buffer,
triggering a KASAN splat:

BUG: KASAN: slab-out-of-bounds in memcmp+0x176/0x1d0 lib/string.c:683
Read of size 8 at addr ffff88810130f5c0 by task syz.1.62/2258

CPU: 2 UID: 0 PID: 2258 Comm: syz.1.62 Not tainted 6.19.0-dirty #3 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0xab/0xe0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xce/0x650 mm/kasan/report.c:482
kasan_report+0xce/0x100 mm/kasan/report.c:595
memcmp+0x176/0x1d0 lib/string.c:683
uuid_equal include/linux/uuid.h:73 [inline]
cxl_payload_from_user_allowed drivers/cxl/core/mbox.c:345 [inline]
cxl_mbox_cmd_ctor drivers/cxl/core/mbox.c:368 [inline]
cxl_validate_cmd_from_user drivers/cxl/core/mbox.c:522 [inline]
cxl_send_cmd+0x9c0/0xb50 drivers/cxl/core/mbox.c:643
__cxl_memdev_ioctl drivers/cxl/core/memdev.c:698 [inline]
cxl_memdev_ioctl+0x14f/0x190 drivers/cxl/core/memdev.c:713
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa8/0x330 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fdaf331ba79
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fdaf1d77038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fdaf3585fa0 RCX: 00007fdaf331ba79
RDX: 00002000000001c0 RSI: 00000000c030ce02 RDI: 0000000000000003
RBP: 00007fdaf33749df R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fdaf3586038 R14: 00007fdaf3585fa0 R15: 00007ffced2af768
</TASK>

Add 'in_size' parameter to cxl_payload_from_user_allowed() and validate
the payload is large enough.