[PATCH] fix a NULL pointer dereference in amdgpu_connector_lcd_native_mode()

[Alex Deucher]

On Tue, Nov 30, 2021 at 6:24 AM Zhou Qingyang <zhou1615 at umn.edu> wrote:
>
> In amdgpu_connector_lcd_native_mode(), the return value of
> drm_mode_duplicate() is assigned to mode, and there is a dereference
> of it in amdgpu_connector_lcd_native_mode(), which will lead to a NULL
> pointer dereference on failure of drm_mode_duplicate().
>
> Fix this bug add 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_AMDGPU=m show no new warnings, and
> our static analyzer no longer warns about this code.
>
> Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
> Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> index 0de66f59adb8..0170aa84c5e6 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> @@ -387,6 +387,9 @@ amdgpu_connector_lcd_native_mode(struct drm_encoder *encoder)
>             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
>