[PATCH 1/2] drm/hyperv: validate resolution_count from host VMBus message

[Berkant Koc <me@berkoc.com>]

The synthetic video device receives a SYNTHVID_RESOLUTION_RESPONSE
containing a u8 resolution_count and a u8 default_resolution_index
from the host. The existing check rejects resolution_count == 0 and
rejects an index that is greater or equal to resolution_count, but
does not bound resolution_count itself against the fixed
supported_resolution[SYNTHVID_MAX_RESOLUTION_COUNT] array. A host
that returns resolution_count > 64 together with an in-range
default_resolution_index causes the subsequent loop to read past
the array.

Reject any resolution_count that exceeds SYNTHVID_MAX_RESOLUTION_COUNT,
folded into the existing zero-check for one log entry per failure.
This matches the input-validation pattern used by other VMBus parsers
under drivers/hv/ and trims one host-controlled value from the trusted
path.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 051ecc526832..003bb118d64c 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -391,8 +391,11 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 		return -ETIMEDOUT;
 	}
 
-	if (msg->resolution_resp.resolution_count == 0) {
-		drm_err(dev, "No supported resolutions\n");
+	if (msg->resolution_resp.resolution_count == 0 ||
+	    msg->resolution_resp.resolution_count >
+	    SYNTHVID_MAX_RESOLUTION_COUNT) {
+		drm_err(dev, "Invalid resolution count: %d\n",
+			msg->resolution_resp.resolution_count);
 		return -ENODEV;
 	}
 
-- 
2.47.3