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

[Rob Herring]

On Thu, Sep 5, 2019 at 1:11 PM Steven Price <steven.price at 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 at 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?

Also, looking at this again, I think we need to hold the mm_lock
around the drm_mm_for_each_node().

Rob