CVE-2026-23163: drm/amdgpu: fix NULL pointer dereference in amdgpu_gmc_filter_faults_remove

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

drm/amdgpu: fix NULL pointer dereference in amdgpugmcfilterfaultsremove

On APUs such as Raven and Renoir (GC 9.1.0, 9.2.2, 9.3.0), the ih1 and ih2 interrupt ring buffers are not initialized. This is by design, as these secondary IH rings are only available on discrete GPUs. See vega10ihswinit() which explicitly skips ih1/ih2 initialization when AMDISAPU is set.

However, amdgpugmcfilterfaultsremove() unconditionally uses ih1 to get the timestamp of the last interrupt entry. When retry faults are enabled on APUs (noretry=0), this function is called from the SVM page fault recovery path, resulting in a NULL pointer dereference when amdgpuihdecodeivtshelper() attempts to access ih->ring[].

The crash manifests as:

BUG: kernel NULL pointer dereference, address: 0000000000000004 RIP: 0010:amdgpuihdecodeivtshelper+0x22/0x40 [amdgpu] Call Trace: amdgpugmcfilterfaultsremove+0x60/0x130 [amdgpu] svmrangerestorepages+0xae5/0x11c0 [amdgpu] amdgpuvmhandlefault+0xc8/0x340 [amdgpu] gmcv90processinterrupt+0x191/0x220 [amdgpu] amdgpuirqdispatch+0xed/0x2c0 [amdgpu] amdgpuihprocess+0x84/0x100 [amdgpu]

This issue was exposed by commit 1446226d32a4 ("drm/amdgpu: Remove GC HW IP 9.3.0 from noretry=1") which changed the default for Renoir APU from noretry=1 to noretry=0, enabling retry fault handling and thus exercising the buggy code path.

Fix this by adding a check for ih1.ringsize before attempting to use it. Also restore the softih support from commit dd299441654f ("drm/amdgpu: Rework retry fault removal"). This is needed if the hardware doesn't support secondary HW IH rings.

v2: additional updates (Alex)

(cherry picked from commit 6ce8d536c80aa1f059e82184f0d1994436b1d526)