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

drm/amdkfd: Don't call mmput from MMU notifier callback

If the process is exiting, the mmput inside mmu notifier callback from
compactd or fork or numa balancing could release the last reference
of mm struct to call exit_mmap and free_pgtable, this triggers deadlock
with below backtrace.

The deadlock will leak kfd process as mmu notifier release is not called
and cause VRAM leaking.

The fix is to take mm reference mmget_non_zero when adding prange to the
deferred list to pair with mmput in deferred list work.

If prange split and add into pchild list, the pchild work_item.mm is not
used, so remove the mm parameter from svm_range_unmap_split and
svm_range_add_child.

The backtrace of hung task:

INFO: task python:348105 blocked for more than 64512 seconds.
Call Trace:
__schedule+0x1c3/0x550
schedule+0x46/0xb0
rwsem_down_write_slowpath+0x24b/0x4c0
unlink_anon_vmas+0xb1/0x1c0
free_pgtables+0xa9/0x130
exit_mmap+0xbc/0x1a0
mmput+0x5a/0x140
svm_range_cpu_invalidate_pagetables+0x2b/0x40 [amdgpu]
mn_itree_invalidate+0x72/0xc0
__mmu_notifier_invalidate_range_start+0x48/0x60
try_to_unmap_one+0x10fa/0x1400
rmap_walk_anon+0x196/0x460
try_to_unmap+0xbb/0x210
migrate_page_unmap+0x54d/0x7e0
migrate_pages_batch+0x1c3/0xae0
migrate_pages_sync+0x98/0x240
migrate_pages+0x25c/0x520
compact_zone+0x29d/0x590
compact_zone_order+0xb6/0xf0
try_to_compact_pages+0xbe/0x220
__alloc_pages_direct_compact+0x96/0x1a0
__alloc_pages_slowpath+0x410/0x930
__alloc_pages_nodemask+0x3a9/0x3e0
do_huge_pmd_anonymous_page+0xd7/0x3e0
__handle_mm_fault+0x5e3/0x5f0
handle_mm_fault+0xf7/0x2e0
hmm_vma_fault.isra.0+0x4d/0xa0
walk_pmd_range.isra.0+0xa8/0x310
walk_pud_range+0x167/0x240
walk_pgd_range+0x55/0x100
__walk_page_range+0x87/0x90
walk_page_range+0xf6/0x160
hmm_range_fault+0x4f/0x90
amdgpu_hmm_range_get_pages+0x123/0x230 [amdgpu]
amdgpu_ttm_tt_get_user_pages+0xb1/0x150 [amdgpu]
init_user_pages+0xb1/0x2a0 [amdgpu]
amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu+0x543/0x7d0 [amdgpu]
kfd_ioctl_alloc_memory_of_gpu+0x24c/0x4e0 [amdgpu]
kfd_ioctl+0x29d/0x500 [amdgpu]

(cherry picked from commit a29e067bd38946f752b0ef855f3dfff87e77bec7)