[PATCH v2] drm/i915/edp: Check supported link rates DPCD read

[Nikita Zhandarovich]

intel_edp_set_sink_rates() reads DP_SUPPORTED_LINK_RATES into a local
stack array and then parses the array unconditionally. If the read
fails, the array contents are not valid and may result in bogus sink
link rates being used.

Use drm_dp_dpcd_read_data() and clear the sink rate array on failure,
so the existing parser falls back to the default sink rate handling.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 68f357cb7347 ("drm/i915/dp: generate and cache sink rate array for all DP, not just eDP 1.4")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
---
v1 -> v2:
- Use drm_dp_dpcd_read_data() instead of drm_dp_dpcd_read().
- Avoid the goto by clearing sink_rates on read failure, as suggested by
  Jani Nikula.
- Adjust patch description.

 drivers/gpu/drm/i915/display/intel_dp.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 6ef2a0043cda..5c3e816b0135 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -4678,10 +4678,17 @@ intel_edp_set_sink_rates(struct intel_dp *intel_dp)
 
 	if (intel_dp->edp_dpcd[0] >= DP_EDP_14) {
 		__le16 sink_rates[DP_MAX_SUPPORTED_RATES];
+		int ret;
 		int i;
 
-		drm_dp_dpcd_read(&intel_dp->aux, DP_SUPPORTED_LINK_RATES,
-				 sink_rates, sizeof(sink_rates));
+		ret = drm_dp_dpcd_read_data(&intel_dp->aux,
+					    DP_SUPPORTED_LINK_RATES,
+					    sink_rates, sizeof(sink_rates));
+		if (ret < 0) {
+			drm_dbg_kms(display->drm,
+				    "Unable to read eDP supported link rates, using default rates\n");
+			memset(sink_rates, 0, sizeof(sink_rates));
+		}
 
 		for (i = 0; i < ARRAY_SIZE(sink_rates); i++) {
 			int rate;