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SubjectRe: [PATCH] drm/panfrost: Prevent race when handling page fault
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;
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel



--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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