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

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

On Sat, 27 Jun 2026, "Kandpal, Suraj" <suraj.kandpal@intel.com> wrote:
>> Subject: [PATCH] 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>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>
> LGTM,
> Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>

Thanks for the review, pushed to din.

BR,
Jani.

>
>> ---
>>  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
>

-- 
Jani Nikula, Intel