Re: [PATCH] drm/i915: Fix CFI violation with show_dynamic_id()

[Sami Tolvanen]

On Thu, May 12, 2022 at 2:17 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> When an attribute group is created with sysfs_create_group(), the
> ->sysfs_ops() callback is set to kobj_sysfs_ops, which sets the ->show()
> callback to kobj_attr_show(). kobj_attr_show() uses container_of() to
> get the ->show() callback from the attribute it was passed, meaning the
> ->show() callback needs to be the same type as the ->show() callback in
> 'struct kobj_attribute'.
>
> However, show_dynamic_id() has the type of the ->show() callback in
> 'struct device_attribute', which causes a CFI violation when opening the
> 'id' sysfs node under drm/card0/metrics. This happens to work because
> the layout of 'struct kobj_attribute' and 'struct device_attribute' are
> the same, so the container_of() cast happens to allow the ->show()
> callback to still work.
>
> Change the type of show_dynamic_id() to match the ->show() callback in
> 'struct kobj_attributes' and update the type of sysfs_metric_id to
> match, which resolves the CFI violation.
>
> Fixes: f89823c21224 ("drm/i915/perf: Implement I915_PERF_ADD/REMOVE_CONFIG interface")
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/i915_perf.c       | 4 ++--
>  drivers/gpu/drm/i915/i915_perf_types.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index 0a9c3fcc09b1..1577ab6754db 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -4050,8 +4050,8 @@ static struct i915_oa_reg *alloc_oa_regs(struct i915_perf *perf,
>         return ERR_PTR(err);
>  }
>
> -static ssize_t show_dynamic_id(struct device *dev,
> -                              struct device_attribute *attr,
> +static ssize_t show_dynamic_id(struct kobject *kobj,
> +                              struct kobj_attribute *attr,
>                                char *buf)
>  {
>         struct i915_oa_config *oa_config =
> diff --git a/drivers/gpu/drm/i915/i915_perf_types.h b/drivers/gpu/drm/i915/i915_perf_types.h
> index 473a3c0544bb..05cb9a335a97 100644
> --- a/drivers/gpu/drm/i915/i915_perf_types.h
> +++ b/drivers/gpu/drm/i915/i915_perf_types.h
> @@ -55,7 +55,7 @@ struct i915_oa_config {
>
>         struct attribute_group sysfs_metric;
>         struct attribute *attrs[2];
> -       struct device_attribute sysfs_metric_id;
> +       struct kobj_attribute sysfs_metric_id;
>
>         struct kref ref;
>         struct rcu_head rcu;

This looks correct to me. Thanks for testing this and fixing the issue!

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Sami