mei: vsc: Fix fortify-panic caused by invalid counted_by() use

gcc 15 honors the __counted_by(len) attribute on vsc_tp_packet.buf[]
and the vsc-tp.c code is using this in a wrong way. len does not contain
the available size in the buffer, it contains the actual packet length
*without* the crc. So as soon as vsc_tp_xfer() tries to add the crc to
buf[] the fortify-panic handler gets triggered:

[   80.842193] memcpy: detected buffer overflow: 4 byte write of buffer size 0
[   80.842243] WARNING: CPU: 4 PID: 272 at lib/string_helpers.c:1032 __fortify_report+0x45/0x50
...
[   80.843175]  __fortify_panic+0x9/0xb
[   80.843186]  vsc_tp_xfer.cold+0x67/0x67 [mei_vsc_hw]
[   80.843210]  ? seqcount_lockdep_reader_access.constprop.0+0x82/0x90
[   80.843229]  ? lockdep_hardirqs_on+0x7c/0x110
[   80.843250]  mei_vsc_hw_start+0x98/0x120 [mei_vsc]
[   80.843270]  mei_reset+0x11d/0x420 [mei]

The easiest fix would be to just drop the counted-by but with the exception
of the ack buffer in vsc_tp_xfer_helper() which only contains enough room
for the packet-header, all other uses of vsc_tp_packet always use a buffer
of VSC_TP_MAX_XFER_SIZE bytes for the packet.

Instead of just dropping the counted-by, split the vsc_tp_packet struct
definition into a header and a full-packet definition and use a fixed
size buf[] in the packet definition, this way fortify-source buffer
overrun checking still works when enabled.

Fixes: 566f5ca ("mei: Add transport driver for IVSC device")
Cc: stable@kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Alexander Usyskin <alexander.usyskin@intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://lore.kernel.org/r/20250318141203.94342-2-hdegoede@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: jwrdegoede

drivers/misc/mei/vsc-tp.c

@@ -36,20 +36,24 @@
 #define VSC_TP_XFER_TIMEOUT_BYTES		700
 #define VSC_TP_PACKET_PADDING_SIZE		1
 #define VSC_TP_PACKET_SIZE(pkt) \
-	(sizeof(struct vsc_tp_packet) + le16_to_cpu((pkt)->len) + VSC_TP_CRC_SIZE)
+	(sizeof(struct vsc_tp_packet_hdr) + le16_to_cpu((pkt)->hdr.len) + VSC_TP_CRC_SIZE)
 #define VSC_TP_MAX_PACKET_SIZE \
-	(sizeof(struct vsc_tp_packet) + VSC_TP_MAX_MSG_SIZE + VSC_TP_CRC_SIZE)
+	(sizeof(struct vsc_tp_packet_hdr) + VSC_TP_MAX_MSG_SIZE + VSC_TP_CRC_SIZE)
 #define VSC_TP_MAX_XFER_SIZE \
 	(VSC_TP_MAX_PACKET_SIZE + VSC_TP_XFER_TIMEOUT_BYTES)
 #define VSC_TP_NEXT_XFER_LEN(len, offset) \
-	(len + sizeof(struct vsc_tp_packet) + VSC_TP_CRC_SIZE - offset + VSC_TP_PACKET_PADDING_SIZE)
+	(len + sizeof(struct vsc_tp_packet_hdr) + VSC_TP_CRC_SIZE - offset + VSC_TP_PACKET_PADDING_SIZE)
 
-struct vsc_tp_packet {
+struct vsc_tp_packet_hdr {
 	__u8 sync;
 	__u8 cmd;
 	__le16 len;
 	__le32 seq;
-	__u8 buf[] __counted_by(len);
+};
+
+struct vsc_tp_packet {
+	struct vsc_tp_packet_hdr hdr;
+	__u8 buf[VSC_TP_MAX_XFER_SIZE - sizeof(struct vsc_tp_packet_hdr)];
 };
 
 struct vsc_tp {
@@ -158,12 +162,12 @@ static int vsc_tp_dev_xfer(struct vsc_tp *tp, void *obuf, void *ibuf, size_t len
 static int vsc_tp_xfer_helper(struct vsc_tp *tp, struct vsc_tp_packet *pkt,
 			      void *ibuf, u16 ilen)
 {
-	int ret, offset = 0, cpy_len, src_len, dst_len = sizeof(struct vsc_tp_packet);
+	int ret, offset = 0, cpy_len, src_len, dst_len = sizeof(struct vsc_tp_packet_hdr);
 	int next_xfer_len = VSC_TP_PACKET_SIZE(pkt) + VSC_TP_XFER_TIMEOUT_BYTES;
 	u8 *src, *crc_src, *rx_buf = tp->rx_buf;
 	int count_down = VSC_TP_MAX_XFER_COUNT;
 	u32 recv_crc = 0, crc = ~0;
-	struct vsc_tp_packet ack;
+	struct vsc_tp_packet_hdr ack;
 	u8 *dst = (u8 *)&ack;
 	bool synced = false;
 
@@ -280,10 +284,10 @@ int vsc_tp_xfer(struct vsc_tp *tp, u8 cmd, const void *obuf, size_t olen,
 
 	guard(mutex)(&tp->mutex);
 
-	pkt->sync = VSC_TP_PACKET_SYNC;
-	pkt->cmd = cmd;
-	pkt->len = cpu_to_le16(olen);
-	pkt->seq = cpu_to_le32(++tp->seq);
+	pkt->hdr.sync = VSC_TP_PACKET_SYNC;
+	pkt->hdr.cmd = cmd;
+	pkt->hdr.len = cpu_to_le16(olen);
+	pkt->hdr.seq = cpu_to_le32(++tp->seq);
 	memcpy(pkt->buf, obuf, olen);
 
 	crc = ~crc32(~0, (u8 *)pkt, sizeof(pkt) + olen);