Re: [PATCH v4 09/18] drm/panthor: Add fine-grained restrictions on VMs

[Adrian Larumbe]

Reviewed-by: Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx>

On 26.08.2026 16:56, Boris Brezillon wrote:
> We currently restrict what a VM is allowed to do based on two states:
> panthor_vm::destroyed and panthor_vm_pgtable::unusable, but we'll soon
> need a no-unmap restriction to fix the unplug logic.
> 
> Instead of adding a third boolean that would reflect this new limitation,
> let's overhaul the current restriction logic by adding separate
> restriction flags representing the operations we want to prevent (map,
> unmap and use).
> 
> Map and use restrictions are set everywhere we were previously
> calling panthor_vm_pgtable_declare_unusable() or setting ::destroyed
> to true, since that's what those two flags were preventing.
> 
> We also add restriction checks in
> panthor_vm_pgtable_prepare_[un]map_op_ctx() and
> panthor_vm_pgtable_exec_op() and drop the ones we had in
> panthor_vm_bind_job_create() since they are redundant.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 134 ++++++++++++++++++++++------------
>  1 file changed, 88 insertions(+), 46 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 933cb820926d..3d9f9bf29e1d 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -225,6 +225,25 @@ struct panthor_as_op_ctx {
>  	} map;
>  };
>  
> +/**
> + * enum panthor_as_restriction - List of restrictions that can apply to an AS.
> + *
> + * An AS always starts unrestricted, but based on the faults or device state
> + * changes, restrictions can be added over time. Restrictions can't be removed
> + * though. Once a VM is restricted, a new one must be created to lift the
> + * restrictions.
> + */
> +enum panthor_as_restriction {
> +	/** @PANTHOR_AS_FORBID_MAP: The AS can't map new buffers. */
> +	PANTHOR_AS_FORBID_MAP = BIT(0),
> +
> +	/** @PANTHOR_AS_FORBID_UNMAP: The AS can't remove existing mappings. */
> +	PANTHOR_AS_FORBID_UNMAP = BIT(1),
> +
> +	/** @PANTHOR_AS_FORBID_USE: The AS can't become active again. */
> +	PANTHOR_AS_FORBID_USE = BIT(2),
> +};
> +
>  /**
>   * struct panthor_as - Used to managed a GPU address space.
>   */
> @@ -285,25 +304,8 @@ struct panthor_as {
>  		struct list_head lru_node;
>  	} hw_slot;
>  
> -	/**
> -	 * @unusable: True if the AS has turned unusable because something
> -	 * bad happened during an asynchronous request.
> -	 *
> -	 * We don't try to recover from such failures, because this implies
> -	 * informing userspace about the specific operation that failed, and
> -	 * hoping the userspace driver can replay things from there. This all
> -	 * sounds very complicated for little gain.
> -	 *
> -	 * Instead, we should just flag the AS as unusable, and fail any
> -	 * further request targeting this AS.
> -	 *
> -	 * We also provide a way to query an AS state, so userspace can
> -	 * destroy it and create a new one.
> -	 *
> -	 * As an analogy, this would be mapped to a VK_ERROR_DEVICE_LOST
> -	 * situation, where the logical device needs to be re-created.
> -	 */
> -	bool unusable;
> +	/** @restrictions: Bitmask of panthor_as_restriction flags. */
> +	atomic_t restrictions;
>  
>  	/**
>  	 * @unhandled_fault: Unhandled fault happened.
> @@ -431,13 +433,6 @@ struct panthor_vm {
>  	/** @for_mcu: True if this is the MCU VM. */
>  	bool for_mcu;
>  
> -	/**
> -	 * @destroyed: True if the VM was destroyed.
> -	 *
> -	 * No further bind requests should be queued to a destroyed VM.
> -	 */
> -	bool destroyed;
> -
>  	/**
>  	 * @dummy: Dummy object used for sparse mappings.
>  	 *
> @@ -699,7 +694,9 @@ bool panthor_vm_has_unhandled_faults(struct panthor_vm *vm)
>   */
>  bool panthor_vm_is_unusable(struct panthor_vm *vm)
>  {
> -	return vm->as->unusable;
> +	return (atomic_read(&vm->as->restrictions) &
> +		(PANTHOR_AS_FORBID_USE | PANTHOR_AS_FORBID_MAP |
> +		 PANTHOR_AS_FORBID_UNMAP));
>  }
>  
>  static void panthor_as_release_hw_slot_locked(struct panthor_as *as)
> @@ -758,6 +755,11 @@ int panthor_vm_active(struct panthor_vm *vm)
>  	mutex_lock(&as->op_lock);
>  	mutex_lock(&ptdev->mmu->as.slots_lock);
>  
> +	if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_USE) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
>  	if (refcount_inc_not_zero(&as->active_cnt))
>  		goto out_unlock;
>  
> @@ -926,21 +928,29 @@ static size_t get_pgsize(u64 addr, size_t size, size_t *count)
>  	return SZ_2M;
>  }
>  
> -static void panthor_as_declare_unusable(struct panthor_as *as)
> +static void panthor_as_restrict_usage_locked(struct panthor_as *as,
> +					     u32 new_restrictions)
>  {
>  	struct panthor_device *ptdev = container_of(as->base.drm, struct panthor_device, base);
>  	int cookie;
>  
> -	if (as->unusable)
> -		return;
> +	lockdep_assert_held(&as->op_lock);
>  
> -	as->unusable = true;
> -	mutex_lock(&ptdev->mmu->as.slots_lock);
> -	if (as->hw_slot.id >= 0 && drm_dev_enter(&ptdev->base, &cookie)) {
> -		panthor_mmu_as_disable(ptdev, as->hw_slot.id, false);
> -		drm_dev_exit(cookie);
> +	if (new_restrictions & PANTHOR_AS_FORBID_USE) {
> +		guard(mutex)(&ptdev->mmu->as.slots_lock);
> +		if (as->hw_slot.id >= 0 && drm_dev_enter(&ptdev->base, &cookie)) {
> +			/* Try to disable the AS. If as_disable() passed, this should cause
> +			 * a fault on the next memory access. If it failed, a reset is
> +			 * scheduled to recover from the GPU hang.
> +			 * We intentionally don't call release_as_locked() here, because
> +			 * this would mess up with the active_cnt refcount.
> +			 */
> +			panthor_mmu_as_disable(ptdev, as->hw_slot.id, false);
> +			drm_dev_exit(cookie);
> +		}
>  	}
> -	mutex_unlock(&ptdev->mmu->as.slots_lock);
> +
> +	atomic_or(new_restrictions, &as->restrictions);
>  }
>  
>  static void panthor_as_unmap_pages(struct panthor_as *as, u64 iova, u64 size)
> @@ -976,7 +986,9 @@ static void panthor_as_unmap_pages(struct panthor_as *as, u64 iova, u64 size)
>  			 * so flag the VM unusable to make sure it's not going
>  			 * to be used anymore.
>  			 */
> -			panthor_as_declare_unusable(as);
> +			panthor_as_restrict_usage_locked(as,
> +							 PANTHOR_AS_FORBID_USE |
> +							 PANTHOR_AS_FORBID_MAP);
>  
>  			/* If we don't make progress, we're screwed. That also means
>  			 * something else prevents us from unmapping the region, but
> @@ -1052,7 +1064,9 @@ panthor_as_map_pages(struct panthor_as *as, u64 iova, int prot,
>  				 * table pages behind.
>  				 */
>  				panthor_as_unmap_pages(as, start_iova, iova - start_iova);
> -				panthor_as_declare_unusable(as);
> +				panthor_as_restrict_usage_locked(as,
> +								 PANTHOR_AS_FORBID_USE |
> +								 PANTHOR_AS_FORBID_MAP);
>  				return ret;
>  			}
>  		}
> @@ -1349,6 +1363,9 @@ static int panthor_as_prepare_map_op_ctx(struct panthor_as_op_ctx *op_ctx,
>  	struct sg_table *sgt = NULL;
>  	int ret;
>  
> +	if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_MAP)
> +		return -EINVAL;
> +
>  	if (!bo)
>  		return -EINVAL;
>  
> @@ -1442,6 +1459,9 @@ static int panthor_as_prepare_unmap_op_ctx(struct panthor_as_op_ctx *op_ctx,
>  	u32 pt_count = 0;
>  	int ret;
>  
> +	if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_UNMAP)
> +		return -EINVAL;
> +
>  	memset(op_ctx, 0, sizeof(*op_ctx));
>  	op_ctx->va.range = size;
>  	op_ctx->va.addr = va;
> @@ -1652,7 +1672,12 @@ static void panthor_vm_destroy(struct panthor_vm *vm)
>  
>  	as = vm->as;
>  	ptdev = container_of(as->base.drm, struct panthor_device, base);
> -	vm->destroyed = true;
> +
> +	scoped_guard(mutex, &as->op_lock) {
> +		panthor_as_restrict_usage_locked(as,
> +						 PANTHOR_AS_FORBID_USE |
> +						 PANTHOR_AS_FORBID_MAP);
> +	}
>  
>  	/* Tell scheduler to stop all GPU work related to this VM */
>  	if (refcount_read(&as->active_cnt) > 0)
> @@ -2169,7 +2194,7 @@ struct panthor_heap_pool *panthor_vm_get_heap_pool(struct panthor_vm *vm, bool c
>  
>  	mutex_lock(&vm->heaps.lock);
>  	if (!vm->heaps.pool && create) {
> -		if (vm->destroyed)
> +		if (panthor_vm_is_unusable(vm))
>  			pool = ERR_PTR(-EINVAL);
>  		else
>  			pool = panthor_heap_pool_create(ptdev, vm);
> @@ -2546,6 +2571,17 @@ int panthor_vm_evict_bo_mappings_locked(struct panthor_gem_object *bo)
>  		if (!mutex_trylock(&as->op_lock))
>  			return -EDEADLK;
>  
> +		/* Unmaps are forbidden when we failed to communicate with the GPU,
> +		 * meaning we can't guarantee that the GPU will see our page table
> +		 * updates which might lead to UAF situations. In that case, we
> +		 * just skip eviction on this VM. Things should go back to normal
> +		 * after a GPU reset.
> +		 */
> +		if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_UNMAP) {
> +			ret = -EBUSY;
> +			goto unlock_op;
> +		}
> +
>  		/* It can be that the vm_bo was already evicted but a new
>  		 * mapping pointing to this BO got created in the meantime,
>  		 * thus turning the vm_bo in partially evicted state. In that case
> @@ -2581,6 +2617,7 @@ int panthor_vm_evict_bo_mappings_locked(struct panthor_gem_object *bo)
>  			vma->evicted = true;
>  		}
>  
> +unlock_op:
>  		mutex_unlock(&as->op_lock);
>  
>  		if (ret)
> @@ -2795,7 +2832,7 @@ static int panthor_as_exec_op(struct panthor_as *as,
>  			.map.gem.offset = op->map.bo_offset,
>  		};
>  
> -		if (as->unusable) {
> +		if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_MAP) {
>  			ret = -EINVAL;
>  			break;
>  		}
> @@ -2805,6 +2842,11 @@ static int panthor_as_exec_op(struct panthor_as *as,
>  	}
>  
>  	case DRM_PANTHOR_VM_BIND_OP_TYPE_UNMAP:
> +		if (atomic_read(&as->restrictions) & PANTHOR_AS_FORBID_UNMAP) {
> +			ret = -EINVAL;
> +			break;
> +		}
> +
>  		ret = drm_gpuvm_sm_unmap(&as->base, as, op->va.addr, op->va.range);
>  		break;
>  
> @@ -2816,8 +2858,11 @@ static int panthor_as_exec_op(struct panthor_as *as,
>  	panthor_as_unlock_region(as);
>  
>  out:
> -	if (ret && flag_vm_unusable_on_failure)
> -		panthor_as_declare_unusable(as);
> +	if (ret && flag_vm_unusable_on_failure) {
> +		panthor_as_restrict_usage_locked(as,
> +						 PANTHOR_AS_FORBID_USE |
> +						 PANTHOR_AS_FORBID_MAP);
> +	}
>  
>  	as->op_ctx = NULL;
>  	mutex_unlock(&as->op_lock);
> @@ -3141,9 +3186,6 @@ panthor_vm_bind_job_create(struct drm_file *file,
>  	if (!vm)
>  		return ERR_PTR(-EINVAL);
>  
> -	if (vm->destroyed || vm->as->unusable)
> -		return ERR_PTR(-EINVAL);
> -
>  	job = kzalloc_obj(*job);
>  	if (!job)
>  		return ERR_PTR(-ENOMEM);
> 
> -- 
> 2.55.0

Adrian Larumbe