drm/amdgpu: fix zero-size GDS range init on RDNA4

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

drm/amdgpu: fix zero-size GDS range init on RDNA4

RDNA4 (GFX 12) hardware removes the GDS, GWS, and OA on-chip memory

resources. The gfx_v12_0 initialisation code correctly leaves

adev->gds.gds_size, adev->gds.gws_size, and adev->gds.oa_size at

zero to reflect this.

amdgpu_ttm_init() unconditionally calls amdgpu_ttm_init_on_chip() for

each of these resources regardless of size. When the size is zero,

amdgpu_ttm_init_on_chip() forwards the call to ttm_range_man_init(),

which calls drm_mm_init(mm, 0, 0). drm_mm_init() immediately fires

DRM_MM_BUG_ON(start + size <= start) -- trivially true when size is

zero -- crashing the kernel during modprobe of amdgpu on an RX 9070 XT.

Guard against this by returning 0 early from

amdgpu_ttm_init_on_chip() when size_in_page is zero. This skips TTM

resource manager registration for hardware resources that are absent,

without affecting any other GPU type.

DRM_MM_BUG_ON() only asserts if CONFIG_DRM_DEBUG_MM is enabled in

the kernel config. This is apparently rarely enabled as these chips

have been in the market for over a year and this issue was only reported

now.

Oops-Analysis: http://oops.fenrus.org/reports/bugzilla.korg/221376/report.html

(cherry picked from commit 5719ce5865279cad4fd5f01011fe037168503f2d)