VERDICT — DF-4605

Bottom line: CONFIRMED (certain) by verbatim-source control-flow reproduction on the DragonFly guest: the port-time #if 0 gutting of the event guard in amdgpu_atif_handler leaves an empty if body, so control falls through to atif->functions.sbios_requests — a load at NULL+0x12 whenever adev->atif == NULL, which is the steady state on every ATIF-less amdgpu system while the ACPI notify handler is installed unconditionally (amdgpu_acpi.c:808-812). Kernel impact: panic (DoS) on the first firmware ACPI_DEVICE_NOTIFY on the GPU node. The proposed one-line fix (restore the early return;) eliminates the fault in the same harness.

1. What was executed on the guest

Guest: DragonFly 6.5-DEVELOPMENT x86_64 (#0 Thu Jul 2 06:02:54 UTC 2026), QEMU/KVM, cc 8.3 [DragonFly]. env.txt records that the guest has no AMD GPU (only virtio-vga 0x1111:1234) and amdgpu is not built into the kernel or module set (sys/conf/files has no amdgpu entries; no /boot/modules). Therefore the in-kernel end-to-end path (firmware notify → AcpiEvNotifyDispatch → amdgpu_acpi_event → panic) cannot be executed on this guest — that is a hardware limitation, not a doubt about the code path.

What CAN be executed faithfully is the control flow itself:

• extract_and_test.sh extracts lines 374–385 (guard) and line 387 (deref) with sed from /usr/src/sys/dev/drm/amd/amdgpu/amdgpu_acpi.c on the guest — the exact bytes of the shipped kernel source;
• compiles them inside a wrapper using the byte-exact struct amdgpu_atif layout from amdgpu_acpi.c:38-75 (verified at runtime: sizeof(struct amdgpu_atif)=40, offsetof(functions.sbios_requests)=18);
• calls the function with a runtime-produced NULL atif and a SIGSEGV handler that prints the faulting address.

Result (run.log, run.stress.log — 4/4 deterministic):

RESULT: SIGSEGV at address 0x12
VERDICT: guard body compiled to NOTHING; control fell through the if() into the NULL deref

The fault address 0x12 (18) matches &((struct amdgpu_atif *)NULL)->functions.sbios_requests exactly. The fixed variant (identical extraction with the #if 0 block replaced by return;) returns cleanly, 4/4.

2. PoC debugging notes (why earlier iterations "passed")

Two harness iterations initially showed no fault, both instructive and neither a refutation:

1. Literal NULL argument (and later a static ... * volatile initialized to NULL): gcc 8.3 constant-propagated the provable NULL through the noinline static function and deleted the call as UB dead code (verified in the disassembly: main loaded the volatile into %rax and never called). In the kernel this is impossible: atif = adev->atif is an opaque heap field read at notify-dispatch time inside ACPICA — not foldable. Fixed by deriving the NULL from getenv() at runtime.
2. Side-effect-free deref body ((void)type;): gcc dropped the entire if (atif->functions.sbios_requests) statement because an empty-bodied if whose condition is a pure load has no observable effect — confirmed even at -O0 (function size 0x4a, no 0x12 displacement anywhere). The kernel body is NOT empty (amdgpu_acpi.c:388-431 calls amdgpu_atif_get_sbios_requests() and the backlight/HPD paths), so the load is emitted in any real build. Fixed by giving the harness body a printf side effect; the fault then appears deterministically.

These are properties of the harness, documented so nobody re-derives them as a refutation. The final harness (both barriers in place) is generated_guard_test.c.

3. Kernel-context reasoning (why 0x12 ⇒ panic, not a benign read)

• The read executes in ACPICA notify-dispatch context (kernel thread); virtual address 0x12 lies in user VA space whose page 0 is unmapped in the dispatching thread's pmap ⇒ kernel-mode page fault ⇒ DragonFly panics ("fault on non-existent ... / segmentation fault") — no SMAP/SMEP needed for this, it is a plain unmapped read.
• No privilege escalation surface: the faulting address is a fixed NULL+0x12 with no attacker-controlled offset; the value read is never used before the trap. Pure DoS.

4. Realistic trigger population

• Any discrete desktop AMD GPU system (no ATIF method in DSDT — the normal desktop firmware state) + any firmware device-notify on the GPU's ACPI node: display hotplug notifications, dock events, PCIe hotplug/PM events, buggy-BIOS notify storms.
• Laptops where amdgpu_atif_verify_interface or the atif kzalloc fails: handler still installed (amdgpu_acpi.c:808-812), adev->atif == NULL.
• Note (functional, not security): on PX/hybrid laptops where ATIF lives in the iGPU namespace, the handler is installed on the dGPU node — events may simply never arrive there; the crash population is chiefly ATIF-less desktops, which is also where amdgpu is most common.

5. Fix validation

• Harness-level: fixed variant returns cleanly 4/4 (baseline faults 4/4).
• Kernel-build level: not testable on this guest — amdgpu is not part of any buildable kernel config here and no AMD GPU exists to probe; fix.diff is authored against the read-only sys/ tree and applies cleanly (git apply --check on the host tree).

6. Relation to prior findings on this file

DF-1749 (stale handler after detach → freed-adev UAF), DF-1750 (4-parser OOB/type-confusion), DF-1751 (allocator mismatch on atif). DF-4605 is independent: live adev, NULL atif, first-notify crash, no teardown required.

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.