Re: [PATCH] drm/amd/display: fix division by zero in get_estimated_bw()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Uncapped request_bw is assigned to allocated_bw, causing an oversized value that leads to an integer underflow during bandwidth validation.
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--- Patch [1]: [PATCH] drm/amd/display: fix division by zero in get_estimated_bw() ---
commit d3870b3b2da2897f934f4033da62b969b6443914
Author: Hari Mishal <harimishal1@gmail.com>

drm/amd/display: fix division by zero in get_estimated_bw()

This commit fixes a division by zero in get_estimated_bw() by returning 0
if bw_granularity is zero. This prevents a crash when a connected DPIA
device reports an estimated bandwidth change before reporting a capability
change.

>     link_dpia_send_bw_alloc_request() already guards the same
>     bw_granularity division; add the identical guard here rather than
>     introducing a new pattern.

[Severity: High]
This is a pre-existing issue, but since the commit message references 
link_dpia_send_bw_alloc_request(), I noticed a potential integer underflow
issue involving allocated_bw.

In link_dpia_send_bw_alloc_request(), the uncapped request_bw variable
is assigned to allocated_bw:

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c:link_dpia_send_bw_alloc_request() {
    ...
		temp = req_bw * link->dpia_bw_alloc_config.bw_granularity;
		request_reg_val = temp / Kbps_TO_Gbps;
		if (temp % Kbps_TO_Gbps)
			++request_reg_val;
	}

	link->dpia_bw_alloc_config.allocated_bw = request_bw;
    ...
}

Later, in link_dpia_validate_dp_tunnel_bandwidth(), remaining_bw is
calculated using unsigned 32-bit subtraction:

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.c:link_dpia_validate_dp_tunnel_bandwidth() {
    ...
			if (router_sets[j].cm_id == dp_tunnel_settings->cm_id) {
				uint32_t remaining_bw =
					dp_tunnel_settings->estimated_bw - dp_tunnel_settings->allocated_bw;
    ...
}

If a connected DPIA display requests more bandwidth than estimated,
allocated_bw will be strictly greater than estimated_bw.

Could this cause the subtraction to underflow to an extremely large
positive value (~4.29 billion), thereby bypassing the subsequent
capacity checks (required_bw > total_bw) and allowing illegal
configurations that oversubscribe the DP tunnel?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260822145751.42328-1-harimishal1@gmail.com?part=1