drm/i915/hdcp: require monotonically increasing seq_num_v

The HDCP 2.2 specification requires the seq_num_v to be monotonically
increasing, and repeated seq_num_v needs to be treated as an integrity
failure. Make it so.

For the first message, seq_num_v must be zero, and is already
checked. We can only check for less-than-or-equal for the subsequent
messages, where hdcp2_encrypted is true.

Discovered using AI-assisted static analysis confirmed by Intel Product
Security.

Reported-by: Martin Hodo <martin.hodo@intel.com>
Fixes: d849178e2c9e ("drm/i915: Implement HDCP2.2 repeater authentication")
Cc: stable@vger.kernel.org # v5.2+
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/20260625104407.1025614-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit 58a224375c81179b52558c53d8857b93196d2687)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_hdcp.c b/drivers/gpu/drm/i915/display/intel_hdcp.c
index 521786a75c42f78ba83cab64b35cd882af2c8c6c..0a076d2ed70afd2b1957c32d732033cb1183f849 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_hdcp.c
+++ b/drivers/gpu/drm/i915/display/intel_hdcp.c
@@ -1801,9 +1801,10 @@ int hdcp2_authenticate_repeater_topology(struct intel_connector *connector)
                return -EINVAL;
        }
 
-       if (seq_num_v < hdcp->seq_num_v) {
-               /* Roll over of the seq_num_v from repeater. Reauthenticate. */
-               drm_dbg_kms(display->drm, "Seq_num_v roll over.\n");
+       if (hdcp->hdcp2_encrypted && seq_num_v <= hdcp->seq_num_v) {
+               /* Reauthenticate on Seq_num_v repeat or rollover */
+               drm_dbg_kms(display->drm, "Seq_num_v %s\n",
+                           seq_num_v == hdcp->seq_num_v ? "repeat" : "rollover");
                return -EINVAL;
        }