Service interruption via uninitialized buffer-clear return

Uninitialized-value use in the Linux kernel allows local users to interrupt service through the AMDGPU buffer-clearing path. In amdgpu_ttm_clear_buffer, the return variable lacks an initial value, so the path where every region is already cleared returns indeterminate status. Reachability requires local low-privileged access and an AMDGPU buffer state where amdgpu_res_cleared is true for every region; the exact availability failure mode is undisclosed.