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

[Nikita Zhandarovich <n.zhandarovich@fintech.ru>]

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 or returns less data than requested, the array contents are not
valid and may result in bogus sink link rates being used.

Check that the full DPCD block was read before parsing it. If not, fall
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@fintech.ru>
---
 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..b6650a12ca54 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -4678,10 +4678,16 @@ 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];
+		ssize_t ret;
 		int i;
 
-		drm_dp_dpcd_read(&intel_dp->aux, DP_SUPPORTED_LINK_RATES,
-				 sink_rates, sizeof(sink_rates));
+		ret = drm_dp_dpcd_read(&intel_dp->aux, DP_SUPPORTED_LINK_RATES,
+				       sink_rates, sizeof(sink_rates));
+		if (ret != sizeof(sink_rates)) {
+			drm_dbg_kms(display->drm,
+				    "Unable to read eDP supported link rates, using default rates\n");
+			goto use_default_rates;
+		}
 
 		for (i = 0; i < ARRAY_SIZE(sink_rates); i++) {
 			int rate;
@@ -4715,6 +4721,7 @@ intel_edp_set_sink_rates(struct intel_dp *intel_dp)
 	 * Use DP_LINK_RATE_SET if DP_SUPPORTED_LINK_RATES are available,
 	 * default to DP_MAX_LINK_RATE and DP_LINK_BW_SET otherwise.
 	 */
+use_default_rates:
 	if (intel_dp->num_sink_rates)
 		intel_dp->use_rate_select = true;
 	else