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

[Christian König]

Am 06.02.23 um 19:21 schrieb Rob Clark:
> On Mon, Feb 6, 2023 at 8:05 AM Christian König <christian.koenig at amd.com> wrote:
>> 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.
> In this case, maybe an dma_resv_assert_held() would be a good idea?

We should already have that. Which makes me really wonder what the heck 
is going on here.

Christian.

>
> BR,
> -R
>
>>> (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);