drm/amdgpu: fix KASAN slab-out-of-bounds in amdgpu_coredump ring dump

Summary

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

drm/amdgpu: fix KASAN slab-out-of-bounds in amdgpu_coredump ring dump

The ring content dump in amdgpu_coredump() uses two separate loops over adev->rings[]: the first counts rings with unsignalled fences to size the allocation, and the second copies ring data into the allocated buffers.

Both loops use the same condition to skip rings:

atomic_read(&ring->fence_drv.last_seq) == ring->fence_drv.sync_seq

Because last_seq is an atomic that is updated concurrently by the fence signalling path, additional rings may appear unsignalled in the second loop that were signalled during the first. When this happens, idx exceeds the allocated ring_count and the store to coredump->rings[idx] writes past the end of the kcalloc-ed buffer.

This was found during IGT stressful test amd_queue_reset which triggers random GPU resets. The OVERSIZE subtest (CMD_STREAM_EXEC_INVALID_PACKET_LENGTH_OVERSIZE on GFX ring) provokes a ring timeout and subsequent coredump, which hits the race between the counting and copying loops. The failure is non-deterministic and depends on fence signalling timing during the reset.

KASAN log:

BUG: KASAN: slab-out-of-bounds in amdgpu_coredump+0x1274/0x12f0 [amdgpu] Write of size 4 at addr ffff888106154258 by task kworker/u128:5/23625 CPU: 16 UID: 0 PID: 23625 Comm: kworker/u128:5 Not tainted 6.19.0+ #35 Workqueue: amdgpu-reset-dev drm_sched_job_timedout [gpu_sched] Call Trace: <TASK> dump_stack_lvl+0xa5/0x110 print_report+0xd1/0x660 kasan_report+0xf3/0x130 __asan_report_store4_noabort+0x17/0x30 amdgpu_coredump+0x1274/0x12f0 [amdgpu] amdgpu_job_timedout+0xef0/0x16c0 [amdgpu] drm_sched_job_timedout+0x194/0x5c0 [gpu_sched] process_one_work+0x84b/0x1990 worker_thread+0x6b8/0x11b0 </TASK>

Allocated by task 23625: kasan_save_stack+0x39/0x70 __kasan_kmalloc+0xc3/0xd0 __kmalloc_noprof+0x2ec/0x910 amdgpu_coredump+0x5c5/0x12f0 [amdgpu] amdgpu_job_timedout+0xef0/0x16c0 [amdgpu]

The buggy address belongs to the object at ffff888106154200 which belongs to the cache kmalloc-rnd-09-96 of size 96 The buggy address is located 16 bytes to the right of allocated 72-byte region [ffff888106154200, ffff888106154248)

72 bytes = 3 * sizeof(struct amdgpu_coredump_ring), so ring_count was 3 but idx reached 3+, writing ring_index (at struct offset 16) 16 bytes past the allocation.

Fix by adding an idx < ring_count guard to the copy loop so it cannot exceed the allocated count even when the fence state changes between the two passes.