linux-kernel - Potential Bug in drm/amd/display/dc

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Date:   Thu, 9 Dec 2021 00:02:35 -0800
From:   Yizhuo Zhai <yzhai003@....edu>
To:     harry.wentland@....com, sunpeng.li@....com,
        Rodrigo.Siqueira@....com, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, amd-gfx@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Potential Bug in drm/amd/display/dc_link

Hi All:
I just found a bug in the cramfs using the static analysis tool, but
not sure if this could happen in reality, could you please advise me
here? Thanks for your attention : ) And please ignore the last one
with HTML format if you did not filter it out.

In function enable_stream_features(), the variable
"old_downspread.raw" could be uninitialized if core_link_read_dpcd
fails(), however, it is used in the later if statement, and further,
core_link_write_dpcd() may write random value, which is potentially
unsafe. But this function does not return the error code to the up
caller and I got stuck in drafting the patch, could you please advise
me here?

The related code:
static void enable_stream_features(struct pipe_ctx *pipe_ctx)
{
     union down_spread_ctrl old_downspread;
    core_link_read_dpcd(link, DP_DOWNSPREAD_CTRL,
                         &old_downspread.raw, sizeof(old_downspread);

        //old_downspread.raw used here
        if (new_downspread.raw != old_downspread.raw) {
               core_link_write_dpcd(link, DP_DOWNSPREAD_CTRL,
                         &new_downspread.raw, sizeof(new_downspread));
        }
}
enum dc_status core_link_read_dpcd(
    struct dc_link *link,
    uint32_t address,
    uint8_t *data,
    uint32_t size)
{
        //data could be uninitialized if the helpers fails and log
some error info
        if (!dm_helpers_dp_read_dpcd(link->ctx,
               link,address, data, size))
                      return DC_ERROR_UNEXPECTED;
        return DC_OK;
}

The same issue in function wait_for_training_aux_rd_interval() in
drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
-- 
Kind Regards,

Yizhuo Zhai

Computer Science, Graduate Student
University of California, Riverside