Re: [PATCH] drm/amdgpu: fix NULL pointer deref in amdgpu_vm_handle_moved for BO-less mappings

On 7/27/26 10:40, Jesse Zhang wrote:
> The individual.needs_update loop in amdgpu_vm_handle_moved() unconditionally
> dereferenced bo_va->base.bo (to read its reservation object, ttm_tt and to test
> for a DMABuf import). However bo_va->base.bo can legitimately be NULL for
> PRT/sparse mappings - amdgpu_vm_bo_update() already handles a NULL bo via its
> "else if (!bo)" PRT path. When such a bo_va reaches the moved list (e.g. the
> userq eviction restore worker running amdgpu_vm_handle_moved() while a user
> queue is being torn down during a GPU reset), the NULL deref crashes the kernel:
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000158
>   #PF: supervisor read access in kernel mode
>   Oops: 0000 [#1] SMP NOPTI
>   Workqueue: events amdgpu_userq_restore_worker [amdgpu]
>   RIP: 0010:amdgpu_vm_handle_moved+0x17a/0x200 [amdgpu]
>   Call Trace:
>    <TASK>
>    amdgpu_userq_vm_validate_and_restore_queue+0x2ce/0x920 [amdgpu]
>    amdgpu_userq_restore_worker+0xce/0x210 [amdgpu]
>    process_scheduled_works+0xa6/0x460
>    worker_thread+0x13c/0x290
>    kthread+0xfb/0x140
>    ret_from_fork+0x1b6/0x2b0
>    ret_from_fork_asm+0x1a/0x30
>    </TASK>
> 
> The faulting instruction is "mov rax,[rdx+0x158]" with rdx (bo) == 0 and
> CR2 == 0x158, i.e. reading bo->tbo.base.resv off a NULL bo.
> 
> Guard the BO-less case: skip the reservation dance and the DMABuf-import check
> when bo is NULL, and let amdgpu_vm_bo_update() take its existing PRT path.

Clear NAK. PRT mappings should *never* be on the moved list in the first place.

Regards,
Christian.

> 
> Signed-off-by: Jesse Zhang <[email protected]>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> index aac8ace9d7a6..fbb76c32bff9 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> @@ -1631,11 +1631,17 @@ int amdgpu_vm_handle_moved(struct amdgpu_device *adev,
>  		bo_va = list_first_entry(&vm->individual.needs_update,
>  					 typeof(*bo_va), base.vm_status);
>  		bo = bo_va->base.bo;
> -		resv = bo->tbo.base.resv;
> +		resv = bo ? bo->tbo.base.resv : NULL;
>  		spin_unlock(&vm->individual_lock);
>  
> +		/* PRT/sparse mappings have no BO to reserve; just update the
> +		 * page tables (amdgpu_vm_bo_update() handles a NULL bo).
> +		 */
> +		if (!bo) {
> +			clear = false;
> +			unlock = false;
>  		/* Try to reserve the BO to avoid clearing its ptes */
> -		if (!adev->debug_vm && !amdgpu_ttm_tt_get_usermm(bo->tbo.ttm) &&
> +		} else if (!adev->debug_vm && !amdgpu_ttm_tt_get_usermm(bo->tbo.ttm) &&
>  		    dma_resv_trylock(resv)) {
>  			clear = false;
>  			unlock = true;
> @@ -1659,7 +1665,7 @@ int amdgpu_vm_handle_moved(struct amdgpu_device *adev,
>  		/* Remember evicted DMABuf imports in compute VMs for later
>  		 * validation
>  		 */
> -		if (vm->is_compute_context &&
> +		if (bo && vm->is_compute_context &&
>  		    drm_gem_is_imported(&bo_va->base.bo->tbo.base) &&
>  		    (!bo_va->base.bo->tbo.resource ||
>  		     bo_va->base.bo->tbo.resource->mem_type == TTM_PL_SYSTEM))