drm/amd/display: Avoid HDCP over-read and corruption

Instead of reading the desired 5 bytes of the actual target field,
the code was reading 8. This could result in a corrupted value if the
trailing 3 bytes were non-zero, so instead use an appropriately sized
and zero-initialized bounce buffer, and read only 5 bytes before casting
to u64.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/modules/hdcp/hdcp1_execution.c

@@ -29,8 +29,10 @@ static inline enum mod_hdcp_status validate_bksv(struct mod_hdcp *hdcp)
 {
 	uint64_t n = 0;
 	uint8_t count = 0;
+	u8 bksv[sizeof(n)] = { };
 
-	memcpy(&n, hdcp->auth.msg.hdcp1.bksv, sizeof(uint64_t));
+	memcpy(bksv, hdcp->auth.msg.hdcp1.bksv, sizeof(hdcp->auth.msg.hdcp1.bksv));
+	n = *(uint64_t *)bksv;
 
 	while (n) {
 		count++;