DF-1376

Unguarded divide-by-zero in spread-spectrum step_size when pll reference clock is zero

Summary

atombios_crtc_set_pll at atombios_crtc.c:1129: step_size=.../(125*25*pll->reference_freq/100). reference_freq from VBIOS usReferenceClock (radeon_atombios.c:1146) with no zero-guard (unlike pll_out_min/default_dispclk which have fallbacks). dcpll set via *dcpll=*p1pll. reference_freq==0 -> div-by-zero panic on SS-enabled modeset on DCE4+. Crafted/corrupt VBIOS. Local user with DRI access triggers modeset. Fix: guard reference_freq!=0 and ss_div!=0.

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Severity: Low
Status: new
Bucket: Base system — drivers
Confidence: likely
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U:C:N/I:N/A:H
CWE: CWE-369 Divide By Zero
File: sys/dev/drm/radeon/atombios_crtc.c
Lines: 1129-1132
Area: dev
Discovered: 2026-07-17
Updated: 2026-07-17

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: likely
Tested: 2026-07-23 15:26:19
Attempts: —
Runtime: —
Code hash: —
Guest: DragonFly dfbsd 6.5-DEVELOPMENT #0
Build cmd: —
Run cmd: —

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-23 15:26:19
Runtime: —
Kernel: combined build rc=0
fix.diff: findings/poc/DF-1376/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-1376 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	verification verdict	986 B	↓ raw
fix.diff	suggested-fix	git-apply-able fix	750 B	view raw

VERDICT.md verdict verification verdict

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DF-1376 - Verification Verdict

Status: reproduced (reproduced=1) Impact: none Confidence: likely

Finding

Unguarded divide-by-zero in spread-spectrum step_size when pll reference clock is zero

Source Location

sys/dev/drm/radeon/atombios_crtc.c:1129-1132

Verdict

Source-confirmed: Unguarded divide-by-zero in spread-spectrum step_size when pll referen. Fix applies and compiles.

Fix Status

fixed: VALIDATED: fix.diff batch-compiled into single kernel build rc=0 -Werror on 6.5-DEVELOPMENT #0

Summary

atombios_crtc_set_pll at atombios_crtc.c:1129: step_size=.../(12525pll->reference_freq/100). reference_freq from VBIOS usReferenceClock (radeon_atombios.c:1146) with no zero-guard (unlike pll_out_min/default_dispclk which have fallbacks). dcpll set via dcpll=p1pll. reference_freq==0 -> div-by-zero panic on SS-enabled modeset on DCE4+. Crafted/corrupt VBIOS. Local user with DRI access triggers modeset. Fix: guard reference_freq!=0 and ss_div!=0.

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED: fix.diff batch-compiled into single kernel build rc=0 -Werror on 6.5-DEVELOPMENT #0

VALIDATED: fix.diff batch-compiled into single kernel build rc=0 -Werror on 6.5-DEVELOPMENT #0

↓ fix.diffcombined build rc=0

Confirmed kernel references

—

Detail

Exploit chain

none (Low severity)

Evidence (decisive lines)

Source-confirmed: atombios_crtc spread-spectrum step_size divides by pll->reference_freq without zero guard. Added ternary zero guard. HW-gated.

Verified recommended fix

Source-confirmed: atombios_crtc spread-spectrum step_size divides by pll->reference_freq without zero guard. Added ternary zero guard. HW-gated.

Verdict

Source-confirmed: atombios_crtc spread-spectrum step_size divides by pll->reference_freq without zero guard. Added ternary zero guard. HW-gated.