[PATCH] drm/i915/hdcp: require monotonically increasing seq_num_v

[Jani Nikula <jani.nikula@intel.com>]

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>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/i915/display/intel_hdcp.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_hdcp.c b/drivers/gpu/drm/i915/display/intel_hdcp.c
index e88fec24af49..d097b478d010 100644
--- a/drivers/gpu/drm/i915/display/intel_hdcp.c
+++ b/drivers/gpu/drm/i915/display/intel_hdcp.c
@@ -1798,9 +1798,10 @@ 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;
 	}
 
-- 
2.47.3