[PATCH] drm/radeon/radeon_connectors: Fix a NULL pointer dereference in radeon_fp_native_mode()

[Alex Deucher]

On Tue, Nov 30, 2021 at 9:49 AM Zhou Qingyang <zhou1615 at umn.edu> wrote:
>
> In radeon_fp_native_mode(), the return value of drm_mode_duplicate() is
> assigned to mode and there is a dereference of it in
> radeon_fp_native_mode(), which could lead to a NULL 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_RADEON=m show no new warnings,
> and our static analyzer no longer warns about this code.
>
> Fixes: d2efdf6d6f42 ("drm/radeon/kms: add cvt mode if we only have lvds w/h and no edid (v4)")
> Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
> ---
>  drivers/gpu/drm/radeon/radeon_connectors.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_connectors.c b/drivers/gpu/drm/radeon/radeon_connectors.c
> index 607ad5620bd9..49f187614f96 100644
> --- a/drivers/gpu/drm/radeon/radeon_connectors.c
> +++ b/drivers/gpu/drm/radeon/radeon_connectors.c
> @@ -473,6 +473,9 @@ static struct drm_display_mode *radeon_fp_native_mode(struct drm_encoder *encode
>             native_mode->vdisplay != 0 &&
>             native_mode->clock != 0) {
>                 mode = drm_mode_duplicate(dev, native_mode);
> +               if (!mode)
> +                       return NULL;
> +

The else if clause needs a similar check.  Care to fix that up as well?

Alex

>                 mode->type = DRM_MODE_TYPE_PREFERRED | DRM_MODE_TYPE_DRIVER;
>                 drm_mode_set_name(mode);
>
> --
> 2.25.1
>