Re: [PATCH] drm/panfrost: Prevent race when handling page fault

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Sep 6, 2019 at 2:42 PM Steven Price <steven.price@arm.com> wrote:
>
> On 06/09/2019 12:10, Rob Herring wrote:
> > On Thu, Sep 5, 2019 at 1:11 PM Steven Price <steven.price@arm.com> wrote:
> >>
> >> When handling a GPU page fault addr_to_drm_mm_node() is used to
> >> translate the GPU address to a buffer object. However it is possible for
> >> the buffer object to be freed after the function has returned resulting
> >> in a use-after-free of the BO.
> >>
> >> Change addr_to_drm_mm_node to return the panfrost_gem_object with an
> >> extra reference on it, preventing the BO from being freed until after
> >> the page fault has been handled.
> >>
> >> Signed-off-by: Steven Price <steven.price@arm.com>
> >> ---
> >>
> >> I've managed to trigger this, generating the following stack trace.
> >
> > Humm, the assumption was that a fault could only happen during a job
> > and so a reference would already be held. Otherwise, couldn't the GPU
> > also be accessing the BO after it is freed?
>
> Ah, I guess I missed that in the commit message. This is assuming that
> user space doesn't include the BO in the job even though the GPU then
> does try to access it. AIUI mesa wouldn't do this, but this is still
> easily possible if user space wants to crash the kernel.

Do we have some nice regression tests for uapi exploits and corner
cases like this? Maybe even in igt?
-Daniel

>
> > Also, looking at this again, I think we need to hold the mm_lock
> > around the drm_mm_for_each_node().
>
> Ah, good point. Squashing in the following should do the trick:
>
> ----8<----
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index ccc536d2e489..41f297aa259c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -396,26 +396,33 @@ static struct panfrost_gem_object *
>  addr_to_drm_mm_node(struct panfrost_device *pfdev, int as, u64 addr)
>  {
>         struct panfrost_gem_object *bo = NULL;
> +       struct panfrost_file_priv *priv;
>         struct drm_mm_node *node;
>         u64 offset = addr >> PAGE_SHIFT;
>         struct panfrost_mmu *mmu;
>
>         spin_lock(&pfdev->as_lock);
>         list_for_each_entry(mmu, &pfdev->as_lru_list, list) {
> -               struct panfrost_file_priv *priv;
> -               if (as != mmu->as)
> -                       continue;
> +               if (as == mmu->as)
> +                       break;
> +       }
> +       if (as != mmu->as)
> +               goto out;
>
> -               priv = container_of(mmu, struct panfrost_file_priv, mmu);
> -               drm_mm_for_each_node(node, &priv->mm) {
> -                       if (offset >= node->start &&
> -                           offset < (node->start + node->size)) {
> -                               bo = drm_mm_node_to_panfrost_bo(node);
> -                               drm_gem_object_get(&bo->base.base);
> -                               goto out;
> -                       }
> +       priv = container_of(mmu, struct panfrost_file_priv, mmu);
> +
> +       spin_lock(&priv->mm_lock);
> +
> +       drm_mm_for_each_node(node, &priv->mm) {
> +               if (offset >= node->start &&
> +                               offset < (node->start + node->size)) {
> +                       bo = drm_mm_node_to_panfrost_bo(node);
> +                       drm_gem_object_get(&bo->base.base);
> +                       break;
>                 }
>         }
> +
> +       spin_unlock(&priv->mm_lock);
>  out:
>         spin_unlock(&pfdev->as_lock);
>         return bo;
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch