High [CVE-2026-64219] Validate payload length and link_index in dc_process_dmub_aux_transfer_async

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Validate payload length and link_index in dc_process_dmub_aux_transfer_async [Why&How] dc_process_dmub_aux_transfer_async() copies payload->length bytes into a 16-byte stack buffer (dpaux.data[16]) guarded only by an ASSERT(), which is a no-op in release builds.

If a caller ever passes length > 16 this results in a stack buffer overflow via memcpy. Additionally, link_index is used to dereference dc->links[] without bounds checking against dc->link_count, risking an out-of-bounds access.

Replace the ASSERT with a hard runtime check that returns false when payload->length exceeds the destination buffer size, and add a bounds check for link_index before it is used. (cherry picked from commit ba4caa9fecdf7a38f98c878ad05a8a64148b6881) This vulnerability is rated as Important rather than Moderate because successful exploitation allows a local user to corrupt kernel memory, potentially achieving elevated privileges or destabilizing the operating system.

Exposure is limited to environments where AMD display hardware is present and the corresponding kernel driver is active, which is uncommon on standard headless enterprise servers. Systems operating without AMD GPU hardware or running virtualized environments without physical GPU pass-through do not execute the vulnerable display engine paths.