Divide-by-zero in spread-spectrum clock math when ATOM BIOS supplies ss.rate==0
Summary
rv730_populate_sclk_value L97-98 and rv730_populate_mclk_value L173-174 compute clk_s=reference_clock*5/(reference_divider*ss.rate) then clk_v=ss.percentage*fbdiv/(clk_s*10000) with no guard against ss.rate==0 or clk_s==0. ss.rate from GPU ATOM BIOS SS table via radeon_atombios_get_asic_ss_info: v1 usSpreadRateInKhz directly radeon_atombios.c:1546; v2/v3 usSpreadRateIn10Hz /=100 at L1569/L1594 -> if usSpreadRateIn10Hz<100 rate becomes 0 after integer divide. Divide-by-zero kernel panic on DPM-enable path radeon_dpm_enable->rv770_convert_power_level_to_smc->rv730_populate_sclk/mclk_value. DPM enabled by default radeon.dpm=1. Attacker: malicious/corrupt VBIOS Thunderbolt/PCIe/VFIO passthrough. Impact: hard kernel panic A:H no C/I. Same pattern in rv770_dpm.c:548 rv740_dpm.c:165 cypress_dpm.c:565 ni_dpm.c:2044 si_dpm.c:4831. Fix: if(ss.rate==0||clk_s==0)goto skip SS.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1937 · 4 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited bug | 1.2 KB | view raw |
| VERDICT.md | verdict | source-confirmation analysis | 1.0 KB | ↓ raw |
| build.sh | build-script | N/A (source-only) | 65 B | view raw |
| run.sh | run-script | N/A (source-only) | 89 B | view raw |
DF-1937 VERDICT
Verdict: REPRODUCED (source-confirmed)
Impact: Low (driver-level NULL deref / OOB / leak / DoS — hardware-gated)
Mechanism: rv730_populate_sclk_value L97-98 and rv730_populate_mclk_value L173-174 compute clk_s=reference_clock5/(reference_dividerss.rate) then clk_v=ss.percentagefbdiv/(clk_s10000) with no guard against s
Citation: sys/dev/drm/radeon/rv730_dpm.c:97-174
Fix: Applied fix.diff — compiles in batch kernel build (rc=0, -Werror).
Verification method: Source-only line-by-line trace of cited path:line. Low-severity driver bug; PoC trigger requires specific hardware or root context. Confirmed the cited vulnerable pattern exists in source.
Fix status (batch validation)
Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest make -j6 nativekernel KERNCONF=X86_64_GENERIC completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.
Fix verification
fixedfix.diff compiled in batch kernel build rc=0 -Werror
fix.diff compiled in batch kernel build rc=0 -Werror
Confirmed kernel references
—
Detail
Exploit chain
none (Low severity)
Evidence (decisive lines)
Source-confirmed: div-by-zero when ss.rate=0 or clk_s=0 in spread-spectrum math (rv730_dpm.c:97-174)
Verified recommended fix
Source-confirmed: div-by-zero when ss.rate=0 or clk_s=0 in spread-spectrum math (rv730_dpm.c:97-174)
Verdict
Source-confirmed: div-by-zero when ss.rate=0 or clk_s=0 in spread-spectrum math (rv730_dpm.c:97-174)
No comments yet.