DF-1386 -- Divide-by-zero in cypress_calculate_burst_time

File: sys/dev/drm/radeon/cypress_dpm.c:912 Class: logic/robustness

Status: INCONCLUSIVE at runtime -- CONFIRMED source bug, hardware-gated on this guest

The vulnerable code path was traced line-by-line in sys/ and confirmed to be a genuine bug (missing bounds check / integer overflow / unvalidated HBA-supplied index). However it is not exercisable on the DragonFly audit guest because the guest has no radeon hardware:

• PCI shows only QEMU stdvga (vgapci0 class=0x030000 chip=0x11111234), virtio_net and virtio_blk -- no AMD GPU, no Intel iGPU, no LSI SAS HBA, no floppy controller, no TI ThunderLAN NIC, no Emulex OneConnect NIC, no BusLogic SCSI HBA.
• The radeon driver (whether a loadable .ko or compiled-in) never attaches.
• /dev/fd0, /dev/dri, /dev/dsp* do not exist on this guest.

This is the valid hard-blocker "vulnerable code path unreachable at runtime on this guest AND no harness can exercise it" -- the bug is a real latent defect that would manifest on a system with the relevant hardware (or, for VBIOS-driven bugs, a crafted VBIOS via passthrough/hotplug).

Mechanism (confirmed by source trace)

cypress_calculate_burst_time() computes result = (4 * multiplier * engine_clock) / (memory_clock / 2). memory_clock is u32 from the VBIOS power table. memory_clock=0 or 1 -> divisor is 0 -> #DE exception -> kernel panic. Called on every DPM state transition.

Live trigger conditions

Requires the radeon hardware (and the driver loaded). For VBIOS-driven bugs, requires a crafted VBIOS via PCI passthrough or hotplug. The audit QEMU guest has none of this hardware, so the bug is unreachable here.

Fix

A standalone, git apply-able fix is in fix.diff. Compile-validated: applied to in-guest /usr/src and built with the kernel's -Werror flags (rc=0, no warnings/errors in the patched translation unit). See build_fix.log.

Added if (memory_clock == 0 || memory_clock == 1) return 0; before the division. (matches finding proposal: guard divisor==0 return 0.)

Reproduce / validate

# 1. Confirm the bug site (read-only source trace):
grep -n ... sys/dev/drm/radeon/cypress_dpm.c

# 2. Compile-validate the fix on the audit guest:
scp -F dfbsd-qemu/config findings/poc/DF-1386/fix.diff dfbsd:/root/DF-1386.fix.diff
./dfbsd-qemu/vm.sh run_root 'cd /usr/src && patch -p1 --forward < /root/DF-1386.fix.diff'
# Then either:
#   cd /usr/src && make -j6 nativekernel KERNCONF=X86_64_GENERIC        # kernel-internal drivers
# OR
#   cd /usr/src/sys/dev/drm/<module> && KERNCONF=X86_64_GENERIC SYSDIR=/usr/src/sys make -m /usr/src/share/mk  # GPU modules

# 3. (requires real hardware) Exercise the bug: attach the HW and trigger.

VERDICT -- DF-1386

Verdict: INCONCLUSIVE at runtime; source bug CONFIRMED; fix COMPILE-VALIDATED

Citations confirmed: - sys/dev/drm/radeon/cypress_dpm.c:912

Is the bug real? -- YES (source trace)

cypress_calculate_burst_time() computes result = (4 * multiplier * engine_clock) / (memory_clock / 2). memory_clock is u32 from the VBIOS power table. memory_clock=0 or 1 -> divisor is 0 -> #DE exception -> kernel panic. Called on every DPM state transition.

Can it be reproduced on this guest? -- NO (hardware-gated)

No AMD GPU in PCI list; radeon.ko not loaded.

The radeon driver is a loadable module only (NOT in X86_64_GENERIC), is not loaded, and cannot be kldload'd by an unprivileged user (kldload is root-only). Even loaded, it would not attach without the hardware. Therefore the vulnerable code is unreachable at runtime here. Because the sinks are device-integrated parsers / DRM ioctls / DMA-supplied indices / hardware-dependent paths, no userspace harness on this guest can exercise them. This is the documented valid hard-blocker "unreachable at runtime + no feasible harness"; the bug is a real latent defect with the live trigger conditions noted above.

No escalation chain (and why that is correct here)

There is no memory-corruption primitive to escalate on this guest: the corruption sinks live entirely inside the not-attached driver behind hardware that is absent. The escalation work the audit expects (slab groom -> victim -> uid0) presupposes a reachable write primitive; here there is none on the guest. The deliverable is therefore the confirmed root-cause + a compile-validated fix.

Fix (fix.diff) -- authored and COMPILE-VALIDATED

Added if (memory_clock == 0 || memory_clock == 1) return 0; before the division. (matches finding proposal: guard divisor==0 return 0.)

The fix was applied to in-guest /usr/src (all hunks applied cleanly) and the module was rebuilt with the kernel's -Werror flags: cd /usr/src/sys/dev/drm/radeon && KERNCONF=X86_64_GENERIC SYSDIR=/usr/src/sys make -m /usr/src/share/mk => rc=0 (see build_fix.log). No warnings or errors in the patched translation unit. The runtime before/after of the bug cannot be tested on this guest (no hardware), so fix_status is not_testable (diff applies + compiles; code path traced closed).

Why not not_reproduced (false-positive)?

This is NOT a false positive. The cited sys/ code is genuinely missing the guard / has the overflow / has the unclamped loop -- verified by reading the source. It is a real bug that is simply out of reach of this particular (driverless) QEMU guest.