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.

Fixes: 6179045 ("cxl/mbox: Block immediate mode in SET_PARTITION_INFO command")
Fixes: 206f9fa ("cxl/mbox: Add Clear Log mailbox command")
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260220001618.963490-2-dave@stgolabs.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Davidlohr Bueso

drivers/cxl/core/mbox.c

@@ -311,6 +311,7 @@ static bool cxl_mem_raw_command_allowed(u16 opcode)
  * cxl_payload_from_user_allowed() - Check contents of in_payload.
  * @opcode: The mailbox command opcode.
  * @payload_in: Pointer to the input payload passed in from user space.
+ * @in_size: Size of @payload_in in bytes.
  *
  * Return:
  *  * true	- payload_in passes check for @opcode.
@@ -325,19 +326,24 @@ static bool cxl_mem_raw_command_allowed(u16 opcode)
  *
  * The specific checks are determined by the opcode.
  */
-static bool cxl_payload_from_user_allowed(u16 opcode, void *payload_in)
+static bool cxl_payload_from_user_allowed(u16 opcode, void *payload_in,
+					  size_t in_size)
 {
 	switch (opcode) {
 	case CXL_MBOX_OP_SET_PARTITION_INFO: {
 		struct cxl_mbox_set_partition_info *pi = payload_in;
 
+		if (in_size < sizeof(*pi))
+			return false;
 		if (pi->flags & CXL_SET_PARTITION_IMMEDIATE_FLAG)
 			return false;
 		break;
 	}
 	case CXL_MBOX_OP_CLEAR_LOG: {
 		const uuid_t *uuid = (uuid_t *)payload_in;
 
+		if (in_size < sizeof(uuid_t))
+			return false;
 		/*
 		 * Restrict the ‘Clear log’ action to only apply to
 		 * Vendor debug logs.
@@ -365,7 +371,8 @@ static int cxl_mbox_cmd_ctor(struct cxl_mbox_cmd *mbox_cmd,
 		if (IS_ERR(mbox_cmd->payload_in))
 			return PTR_ERR(mbox_cmd->payload_in);
 
-		if (!cxl_payload_from_user_allowed(opcode, mbox_cmd->payload_in)) {
+		if (!cxl_payload_from_user_allowed(opcode, mbox_cmd->payload_in,
+						  in_size)) {
 			dev_dbg(cxl_mbox->host, "%s: input payload not allowed\n",
 				cxl_mem_opcode_to_name(opcode));
 			kvfree(mbox_cmd->payload_in);