drm/amd/display: Fix dc_link NULL handling in HPD init

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix dc_link NULL handling in HPD init amdgpu_dm_hpd_init() may see connectors without a valid dc_link. The code already checks dc_link for the polling decision, but later unconditionally dereferences it when setting up HPD interrupts. Assign dc_link early and skip connectors where it is NULL. Fixes the below: drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_irq.c:940 amdgpu_dm_hpd_init() error: we previously assumed 'dc_link' could be null (see line 931) drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_irq.c 923 /* 924 * Analog connectors may be hot-plugged unlike other connector 925 * types that don't support HPD. Only poll analog connectors. 926 */ 927 use_polling |= 928 amdgpu_dm_connector->dc_link && ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The patch adds this NULL check but hopefully it can be removed 929 dc_connector_supports_analog(amdgpu_dm_connector->dc_link->link_id.id); 930 931 dc_link = amdgpu_dm_connector->dc_link; dc_link assigned here. 932 933 /* 934 * Get a base driver irq reference for hpd ints for the lifetime 935 * of dm. Note that only hpd interrupt types are registered with 936 * base driver; hpd_rx types aren't. IOW, amdgpu_irq_get/put on 937 * hpd_rx isn't available. DM currently controls hpd_rx 938 * explicitly with dc_interrupt_set() 939 */ --> 940 if (dc_link->irq_source_hpd != DC_IRQ_SOURCE_INVALID) { ^^^^^^^^^^^^^^^^^^^^^^^ If it's NULL then we are trouble because we dereference it here. 941 irq_type = dc_link->irq_source_hpd - DC_IRQ_SOURCE_HPD1; 942 /* 943 * TODO: There's a mismatch between mode_info.num_hpd 944 * and what bios reports as the # of connectors with hpd