usb: typec: wcove: don't write past struct pd_message in wcove_read_rx_buffer()

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

usb: typec: wcove: don't write past struct pd_message in wcove_read_rx_buffer()

wcove_read_rx_buffer() copies the PD RX FIFO into the caller's

struct pd_message with

for (i = 0; i < USBC_RXINFO_RXBYTES(info); i++)

regmap_read(wcove->regmap, USBC_RX_DATA + i, msg + i);

which has two problems:

USBC_RXINFO_RXBYTES() is a 5-bit field (max 31) while struct pd_message

is 30 bytes (__le16 header + __le32 payload[PD_MAX_PAYLOAD], packed).

The byte count latched in RXINFO is the number of bytes the port partner

put on the wire, so a malicious partner that transmits a 31-byte frame

can drive the loop one byte past the destination if the WCOVE BMC

receiver does not enforce the PD object-count limit in hardware. The

existing FIXME flagged this as unverified.

Independently, regmap_read() takes an unsigned int * and stores a full

unsigned int at the destination. Passing the byte pointer msg + i means

each iteration writes four bytes; the high three are zero (val_bits is

8) and are normally overwritten by the next iteration, but the final

iteration's high bytes are not. With RXBYTES == 30 the i == 29 iteration

already writes three zero bytes past msg, which sits on the IRQ thread's

stack in wcove_typec_irq().

Clamp the loop to sizeof(struct pd_message) and read each register into

a local before storing only its low byte, so the copy can never exceed

the destination regardless of what RXINFO reports.