[PATCH] drm/amd/display: Fix a NULL pointer dereference in amdgpu_dm_connector_add_common_modes()

[Alex Deucher]

Applied.  Thanks!

Alex

On Mon, Jan 24, 2022 at 12:05 PM Zhou Qingyang <zhou1615 at umn.edu> wrote:
>
> In amdgpu_dm_connector_add_common_modes(), amdgpu_dm_create_common_mode()
> is assigned to mode and is passed to drm_mode_probed_add() directly after
> that. drm_mode_probed_add() passes &mode->head to list_add_tail(), and
> there is a dereference of it in list_add_tail() without recoveries, which
> could lead to NULL pointer dereference on failure of
> amdgpu_dm_create_common_mode().
>
> Fix this by adding a NULL check of mode.
>
> This bug was found by a static analyzer.
>
> Builds with 'make allyesconfig' show no new warnings,
> and our static analyzer no longer warns about this code.
>
> Fixes: e7b07ceef2a6 ("drm/amd/display: Merge amdgpu_dm_types and amdgpu_dm")
> Signed-off-by: Zhou Qingyang <zhou1615 at umn.edu>
> ---
> 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.
>
>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index 7f9773f8dab6..9ad94186b146 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -8143,6 +8143,9 @@ static void amdgpu_dm_connector_add_common_modes(struct drm_encoder *encoder,
>                 mode = amdgpu_dm_create_common_mode(encoder,
>                                 common_modes[i].name, common_modes[i].w,
>                                 common_modes[i].h);
> +               if (!mode)
> +                       continue;
> +
>                 drm_mode_probed_add(connector, mode);
>                 amdgpu_dm_connector->num_modes++;
>         }
> --
> 2.25.1
>