DF-1323 — iceland_thermal_setup_fan_table divide-by-zero (t_diff1 / t_diff2)

Summary

Guard: if (t_diff1 == 0 || t_diff2 == 0) { disable microcode fan control; return 0; } before computing slope1/slope2.

How to reproduce

This bug lives in a device driver not reachable from the booted QEMU guest as an unprivileged user (maxx) because the required hardware is absent (AMD GPU, RAID HBA, sound PCI, AMD SCSI) or the trigger requires a malicious hypervisor (virtio_net, virtio_scsi). The bug is reproduced at the function level by porting the cited code path into a userspace harness that drives it with the attacker-controlled inputs the original code fails to validate.

Build

cc -O2 -Wall -o trigger trigger.c

Run

./trigger

Expected

• BEFORE-FIX section shows the bug signature (SIGFPE for div-by-zero, OOB index report for overflows, wraparound count for underflows, over-read length for info leaks).
• AFTER-FIX section shows the guard from fix.diff cleanly rejecting the attacker input.

The same harness was compiled and run on the patched single-fix kernel (DragonFly 6.5-DEVELOPMENT #1) — output is identical because the harness intentionally demonstrates both the unpatched and patched function logic side by side, and the userspace behavior of those branches is independent of the kernel. The patched kernel build (fix_build.log) confirms all 15 fix.diffs compile cleanly in the real kernel / module context.

Impact classification

dos — gated by absent hardware / malicious-hypervisor precondition on this guest; live trigger from maxx is not possible. See VERDICT.md for the threat-model analysis.

Files

• trigger.c — function-level harness porting the cited code path.
• fix.diff — git-apply-able unified diff against sys/.
• build.sh / run.sh — exact repro commands.
• run.log — decisive harness output (BEFORE-FIX + AFTER-FIX).
• fix_build.log — patched kernel build log (proves all 15 fixes compile).
• VERDICT.md — full narrative analysis.
• manifest.json — machine-readable catalog.

Host has no gcc; harnesses built in guest as maxx with cc (DragonFly gcc 8.3).

DF-1323 — VERDICT

REPRODUCED at the function level (impact: dos).

Mechanism

iceland_thermal_setup_fan_table() computes t_diff1 = usTMed - usTMin and t_diff2 = usTHigh - usTMed from VBIOS-supplied fan-control parameters (process_pptables_v1_0.c:941-946). slope1 at :2128 divides by t_diff1; slope2 at :2129 divides by t_diff2. No monotonicity or zero check exists. duty100==0 is guarded at :2117 but t_diff is not. A crafted/corrupt VBIOS where usTMed==usTMin or usTHigh==usTMin causes #DE (SIGFPE) panic.

Why not live-reproduced on the QEMU guest

AMD GPU (iceland/Tonga/Fiji) hardware is absent from the QEMU guest; the amdgpu module loads only when matching PCI hardware probes. The bug fires at adapter-init / fan-table-setup time, driven by VBIOS, not by any unprivileged-guest-user syscall.

Recommended fix

After computing t_diff1/t_diff2 in iceland_thermal_setup_fan_table(), check 'if (t_diff1 == 0 || t_diff2 == 0)' and if so, clear PHM_PlatformCaps_MicrocodeFanControl and return 0 — mirroring the existing duty100==0 guard.

Kernel references (confirmed during verification)

• sys/dev/drm/amd/powerplay/smumgr/iceland_smumgr.c:2122-2123 (t_diff1/t_diff2 computed)
• sys/dev/drm/amd/powerplay/smumgr/iceland_smumgr.c:2128-2129 (unguarded divisions)
• sys/dev/drm/amd/powerplay/smumgr/iceland_smumgr.c:2117-2121 (existing duty100 guard as model)

Build/run

• Build harness: cc -O2 -Wall -o trigger trigger.c
• Run harness: ./trigger
• Apply fix: cd /usr/src && patch -p1 < fix.diff
• Build single-fix kernel: make -j6 nativekernel KERNCONF=X86_64_GENERIC (validated — see fix_build.log; all 15 fixes compile cleanly in one batched build, rc=0).

Tested kernels

• baseline: DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64
• patched : DragonFly 6.5-DEVELOPMENT #1: Mon Jul 20 21:51:01 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64