CVE-2024-47720: drm/amd/display: Add null check for set_output_gamma in dcn30_set_output_transfer_func
In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add null check for set_output_gamma in dcn30_set_output_transfer_func

This commit adds a null check for the set_output_gamma function pointer
in the  dcn30_set_output_transfer_func function. Previously,
set_output_gamma was being checked for nullity at line 386, but then it
was being dereferenced without any nullity check at line 401. This
could potentially lead to a null pointer dereference error if
set_output_gamma is indeed null.

To fix this, we now ensure that set_output_gamma is not null before
dereferencing it. We do this by adding a nullity check for
set_output_gamma before the call to set_output_gamma at line 401. If
set_output_gamma is null, we log an error message and do not call the
function.

This fix prevents a potential null pointer dereference error.

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn30/dcn30_hwseq.c:401 dcn30_set_output_transfer_func()
error: we previously assumed 'mpc->funcs->set_output_gamma' could be null (see line 386)

drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn30/dcn30_hwseq.c
    373 bool dcn30_set_output_transfer_func(struct dc *dc,
    374                                 struct pipe_ctx *pipe_ctx,
    375                                 const struct dc_stream_state *stream)
    376 {
    377         int mpcc_id = pipe_ctx->plane_res.hubp->inst;
    378         struct mpc *mpc = pipe_ctx->stream_res.opp->ctx->dc->res_pool->mpc;
    379         const struct pwl_params *params = NULL;
    380         bool ret = false;
    381
    382         /* program OGAM or 3DLUT only for the top pipe*/
    383         if (pipe_ctx->top_pipe == NULL) {
    384                 /*program rmu shaper and 3dlut in MPC*/
    385                 ret = dcn30_set_mpc_shaper_3dlut(pipe_ctx, stream);
    386                 if (ret == false && mpc->funcs->set_output_gamma) {
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ If this is NULL

    387                         if (stream->out_transfer_func.type == TF_TYPE_HWPWL)
    388                                 params = &stream->out_transfer_func.pwl;
    389                         else if (pipe_ctx->stream->out_transfer_func.type ==
    390                                         TF_TYPE_DISTRIBUTED_POINTS &&
    391                                         cm3_helper_translate_curve_to_hw_format(
    392                                         &stream->out_transfer_func,
    393                                         &mpc->blender_params, false))
    394                                 params = &mpc->blender_params;
    395                          /* there are no ROM LUTs in OUTGAM */
    396                         if (stream->out_transfer_func.type == TF_TYPE_PREDEFINED)
    397                                 BREAK_TO_DEBUGGER();
    398                 }
    399         }
    400
--> 401         mpc->funcs->set_output_gamma(mpc, mpcc_id, params);
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Then it will crash

    402         return ret;
    403 }
CVSS v3.1: 5.5