[PATCH 6.12.y 2/2] drm/i915/hdcp: require monotonically increasing seq_num_v

From: Jani Nikula <[email protected]>

[ Upstream commit db9e64c983dcb07ff256bd455f258c44aa530ff8 ]

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 <[email protected]>
Fixes: d849178e2c9e ("drm/i915: Implement HDCP2.2 repeater authentication")
Cc: [email protected] # v5.2+
Cc: Suraj Kandpal <[email protected]>
Reviewed-by: Suraj Kandpal <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jani Nikula <[email protected]>
(cherry picked from commit 58a224375c81179b52558c53d8857b93196d2687)
Signed-off-by: Joonas Lahtinen <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
 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 565880d3a558..a72adce51dd4 100644
--- a/drivers/gpu/drm/i915/display/intel_hdcp.c
+++ b/drivers/gpu/drm/i915/display/intel_hdcp.c
@@ -1742,9 +1742,10 @@ 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;
 	}
 
-- 
2.53.0