DF-1129

Divide-by-zero in si_thermal_setup_fan_table from unvalidated BIOS fan-table temperatures

Summary

si_thermal_setup_fan_table at si_dpm.c:6489-6496: t_diff1=t_med-t_min and t_diff2=t_high-t_med from BIOS fan table used as divisors in slope1/slope2 with no zero guard. If t_med==t_min or t_high==t_med: integer divide-by-zero -> kernel #DE trap -> panic during DPM enable. Same class at si_calculate_sclk_params:5290 (/reference_divider*ss.rate) and si_populate_mclk_value:5390 (/ss.rate). Requires VBIOS with equal thermal breakpoints via VFIO romfile. Fix: guard divisors and degrade gracefully.

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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:H/UI:N/S:U:C:N/I:N/A:H
CWE: CWE-369 Divide By Zero
File: sys/dev/drm/amd/amdgpu/si_dpm.c
Lines: 6489-6496
Area: dev
Discovered: 2026-07-15
Updated: 2026-07-15

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: panic
Confidence: certain
Tested: 2026-07-25 10:25:03
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: certain
Tested: 2026-07-25 10:25:03
Runtime: —
Kernel: combined build rc=0
fix.diff: findings/poc/DF-1129/fix.diff
Runner: df-bsd-pocrunner

jump to fix verification ↓

PoC verification

Evidence pack

findings/poc/DF-1129 · 3 files

File	Type	Description	Size
VERDICT.md	verdict	Source-confirmation verdict for DF-1129	619 B	↓ raw
fix.diff	suggested-fix	Guard zero divisors in si_thermal_setup_fan_table	485 B	view raw
../fix_build.log	build-log	Batch kernel build log (all fixes, rc=0)	5.6 MB	↓ download

VERDICT.md verdict Source-confirmation verdict for DF-1129

↓ download raw

DF-1129 Verification Verdict

Severity: Low Impact class: panic Verification method: Source-only confirmation (HW-gated, not triggerable on QEMU guest)

Verdict: REPRODUCED (source-confirmed)

The bug is confirmed in the audited source at the cited path:line. Triggerable but requires specific driver/config.

Fix: Guard zero divisors in si_thermal_setup_fan_table

Fix applied and validated in batch kernel build (rc=0, -Werror).

Fix validation

All 41-fix patches batched into single make -j6 nativekernel KERNCONF=X86_64_GENERIC build. Build result: rc=0, 0 errors (full -Werror clean).

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED: fix.diff applies cleanly + batch kernel build rc=0 -Werror; bug HW/module/runtime-gated, no runtime PoC re-test possible on guest.

VALIDATED: fix.diff applies cleanly + batch kernel build rc=0 -Werror; bug HW/module/runtime-gated, no runtime PoC re-test possible on guest.

↓ fix.diffcombined build rc=0

Confirmed kernel references

—

Detail

Exploit chain

none

Evidence (decisive lines)

REPRODUCED (source-only): si_thermal_setup_fan_table uses t_diff1=t_med-t_min and t_diff2=t_high-t_med as divisors in slope1/slope2 with no zero guard; if BIOS table has equal values -> #DE panic.

Verified recommended fix

REPRODUCED (source-only): si_thermal_setup_fan_table uses t_diff1=t_med-t_min and t_diff2=t_high-t_med as divisors in slope1/slope2 with no zero guard; if BIOS table has equal values -> #DE panic.

Verdict

REPRODUCED (source-only): si_thermal_setup_fan_table uses t_diff1=t_med-t_min and t_diff2=t_high-t_med as divisors in slope1/slope2 with no zero guard; if BIOS table has equal values -> #DE panic.