[PATCH] drm/amdgpu: Fix potential race processing vm->freed

[Christian König]

Am 06.02.23 um 16:52 schrieb Rob Clark:
> On Mon, Feb 6, 2023 at 2:15 AM Christian König <christian.koenig at amd.com> wrote:
>> Am 03.02.23 um 19:10 schrieb Rob Clark:
>>> From: Rob Clark <robdclark at chromium.org>
>>>
>>> If userspace calls the AMDGPU_CS ioctl from multiple threads, because
>>> the vm is global to the drm_file, you can end up with multiple threads
>>> racing in amdgpu_vm_clear_freed().  So the freed list should be
>>> protected with the status_lock, similar to other vm lists.
>> Well this is nonsense. To process the freed list the VM root PD lock
>> must be held anyway.
>>
>> If we have a call path where this isn't true then we have a major bug at
>> a different place here.
> I'm not super familiar w/ the amdgpu cs parser stuff, but the only
> thing that I'm seeing that protects things is the bo_list_mutex and it
> isn't clear to me that this is 1:1 with the vm (it looks like it is
> not).

Do you have a backtrace?

Take a look at the reservation object of vm->root.bo. This should always 
be locked first before doing *anything* in a CS.

If that isn't the case we have a much worse problem.

> (I cc'd you on the bug report, jfyi)

I unfortunately only get a permission denied when I try to access that one.

Regards,
Christian.

>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>> Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
>>> Signed-off-by: Rob Clark <robdclark at chromium.org>
>>> ---
>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 33 ++++++++++++++++++++++----
>>>    1 file changed, 29 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
>>> index b9441ab457ea..aeed7bc1512f 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
>>> @@ -1240,10 +1240,19 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
>>>        struct amdgpu_bo_va_mapping *mapping;
>>>        uint64_t init_pte_value = 0;
>>>        struct dma_fence *f = NULL;
>>> +     struct list_head freed;
>>>        int r;
>>>
>>> -     while (!list_empty(&vm->freed)) {
>>> -             mapping = list_first_entry(&vm->freed,
>>> +     /*
>>> +      * Move the contents of the VM's freed list to a local list
>>> +      * that we can iterate without racing against other threads:
>>> +      */
>>> +     spin_lock(&vm->status_lock);
>>> +     list_replace_init(&vm->freed, &freed);
>>> +     spin_unlock(&vm->status_lock);
>>> +
>>> +     while (!list_empty(&freed)) {
>>> +             mapping = list_first_entry(&freed,
>>>                        struct amdgpu_bo_va_mapping, list);
>>>                list_del(&mapping->list);
>>>
>>> @@ -1258,6 +1267,15 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
>>>                amdgpu_vm_free_mapping(adev, vm, mapping, f);
>>>                if (r) {
>>>                        dma_fence_put(f);
>>> +
>>> +                     /*
>>> +                      * Move any unprocessed mappings back to the freed
>>> +                      * list:
>>> +                      */
>>> +                     spin_lock(&vm->status_lock);
>>> +                     list_splice_tail(&freed, &vm->freed);
>>> +                     spin_unlock(&vm->status_lock);
>>> +
>>>                        return r;
>>>                }
>>>        }
>>> @@ -1583,11 +1601,14 @@ int amdgpu_vm_bo_unmap(struct amdgpu_device *adev,
>>>        mapping->bo_va = NULL;
>>>        trace_amdgpu_vm_bo_unmap(bo_va, mapping);
>>>
>>> -     if (valid)
>>> +     if (valid) {
>>> +             spin_lock(&vm->status_lock);
>>>                list_add(&mapping->list, &vm->freed);
>>> -     else
>>> +             spin_unlock(&vm->status_lock);
>>> +     } else {
>>>                amdgpu_vm_free_mapping(adev, vm, mapping,
>>>                                       bo_va->last_pt_update);
>>> +     }
>>>
>>>        return 0;
>>>    }
>>> @@ -1671,7 +1692,9 @@ int amdgpu_vm_bo_clear_mappings(struct amdgpu_device *adev,
>>>                    tmp->last = eaddr;
>>>
>>>                tmp->bo_va = NULL;
>>> +             spin_lock(&vm->status_lock);
>>>                list_add(&tmp->list, &vm->freed);
>>> +             spin_unlock(&vm->status_lock);
>>>                trace_amdgpu_vm_bo_unmap(NULL, tmp);
>>>        }
>>>
>>> @@ -1788,7 +1811,9 @@ void amdgpu_vm_bo_del(struct amdgpu_device *adev,
>>>                amdgpu_vm_it_remove(mapping, &vm->va);
>>>                mapping->bo_va = NULL;
>>>                trace_amdgpu_vm_bo_unmap(bo_va, mapping);
>>> +             spin_lock(&vm->status_lock);
>>>                list_add(&mapping->list, &vm->freed);
>>> +             spin_unlock(&vm->status_lock);
>>>        }
>>>        list_for_each_entry_safe(mapping, next, &bo_va->invalids, list) {
>>>                list_del(&mapping->list);