re: drm/amd/display: Add HDCP module - static analysis bug report

[Colin Ian King <colin.king@canonical.com>]

Hi,

Static analysis with Coverity has detected a potential issue with
function validate_bksv in
drivers/gpu/drm/amd/display/modules/hdcp/hdcp1_execution.c with recent
commit:

commit ed9d8e2bcb003ec94658cafe9b1bb3960e2139ec
Author: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Date:   Tue Aug 6 17:52:01 2019 -0400

    drm/amd/display: Add HDCP module


The analysis is as follows:

 28 static inline enum mod_hdcp_status validate_bksv(struct mod_hdcp *hdcp)
 29 {

CID 89852 (#1 of 1): Out-of-bounds read (OVERRUN)

1. overrun-local:
Overrunning array of 5 bytes at byte offset 7 by dereferencing pointer
(uint64_t *)hdcp->auth.msg.hdcp1.bksv.

 30        uint64_t n = *(uint64_t *)hdcp->auth.msg.hdcp1.bksv;
 31        uint8_t count = 0;
 32
 33        while (n) {
 34                count++;
 35                n &= (n - 1);
 36        }

hdcp->auth.msg.hdcp1.bksv is an array of 5 uint8_t as defined in
drivers/gpu/drm/amd/display/modules/hdcp/hdcp.h as follows:

struct mod_hdcp_message_hdcp1 {
        uint8_t         an[8];
        uint8_t         aksv[5];
        uint8_t         ainfo;
        uint8_t         bksv[5];
        uint16_t        r0p;
        uint8_t         bcaps;
        uint16_t        bstatus;
        uint8_t         ksvlist[635];
        uint16_t        ksvlist_size;
        uint8_t         vp[20];

        uint16_t        binfo_dp;
};

variable n is going to contain the contains of r0p and bcaps. I'm not
sure if that is intentional. If not, then the count is going to be
incorrect if these are non-zero.

Colin