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

[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?

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);
>