drm/amd/display: guard dc_sink dereferences in MST mode validation

Overview

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: guard dc_sink dereferences in MST mode validation

dm_dp_mst_is_port_support_mode() reads aconnector->dc_sink->dsc_caps... for the DSC branch-throughput check, and get_conv_frl_bw()'s HDMI-PCON FRL-bandwidth path reads aconnector->dc_sink->edid_caps.max_frl_rate, both without a NULL check. dc_sink is cleared asynchronously on MST unplug, and both functions run from paths that the driver's own comments document as racing that teardown: the connector probe worker's ->mode_valid callback and a compositor's atomic check, neither of which holds the MST manager lock that the teardown path uses. The former does have an existing dsc_aux NULL check, but dsc_aux isn't reliably cleared in every path that clears dc_sink, so it doesn't cover this.

Fail the port-support check and skip the FRL conversion path when the sink is already gone.

(cherry picked from commit 7c3db8da4e039698ae198c870712428644e965c7)

Affected

• Linux >= f04d275d94e164d24516c32437c988a323018f63 < daa0437fe5e451856aaf8fda602a9fc2105bbd54
• Linux >= f04d275d94e164d24516c32437c988a323018f63 < 217f64f348c90cba62164e7ff38e0ccf78a5ca09
• Linux 6.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.