[PATCH] drm/gma500/psb_intel_lvds: Fix a wild pointer dereference in psb_intel_lvds_get_modes()

[Patrik Jakobsson]

On Tue, Nov 30, 2021 at 2:30 PM Zhou Qingyang <zhou1615 at umn.edu> wrote:
>
> In psb_intel_lvds_get_modes(), the return value of drm_mode_duplicate()
> is assigned to mode and used in drm_mode_probed_add().
> drm_mode_probed_add() passes mode->head to list_add_tail().
> list_add_tail() will further call __list_add() and there is a
> dereference of mode->head in __list_add(), which could lead to a wild
> pointer dereference on failure of drm_mode_duplicate().
>
> Fix this bug by adding a check of mode.
>
> This bug was found by a static analyzer. The analysis employs
> differential checking to identify inconsistent security operations
> (e.g., checks or kfrees) between two code paths and confirms that the
> inconsistent operations are not recovered in the current function or
> the callers, so they constitute bugs.
>
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
>
> Builds with CONFIG_DRM_GMA500=m show no new warnings,
> and our static analyzer no longer warns about this code.
>
> Fixes: 89c78134cc54 ("gma500: Add Poulsbo support")
> Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
> ---
>  drivers/gpu/drm/gma500/psb_intel_lvds.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/gma500/psb_intel_lvds.c b/drivers/gpu/drm/gma500/psb_intel_lvds.c
> index ac97e0d3c7dd..da928189be91 100644
> --- a/drivers/gpu/drm/gma500/psb_intel_lvds.c
> +++ b/drivers/gpu/drm/gma500/psb_intel_lvds.c
> @@ -505,6 +505,9 @@ static int psb_intel_lvds_get_modes(struct drm_connector *connector)
>         if (mode_dev->panel_fixed_mode != NULL) {
>                 struct drm_display_mode *mode =
>                     drm_mode_duplicate(dev, mode_dev->panel_fixed_mode);
> +               if (!mode)
> +                       return -ENOMEM;
> +

Hi, Zhou.

psb_intel_lvds_get_modes() implements .get_modes in struct
drm_connector_helper_funcs. It should return the number of added modes
and not an error code.

-Patrik

>                 drm_mode_probed_add(connector, mode);
>                 return 1;
>         }
> --
> 2.25.1
>