DF-1330 — envy24htmixer_setrecsrc OOB array index from bitmask

Summary

Convert src bitmask to index via ffs(src)-1 with bounds/ single-bit check; reject multi-bit/OOB inputs.

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

leak — 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-1330 — VERDICT

REPRODUCED at the function level (impact: leak).

Mechanism

envy24htmixer_setrecsrc(m, src) at envy24ht.c:1956 indexes envy24ht_mixmap[src]. src is a bitmask masked to ENVY24HT_MIX_REC_MASK=0x3e0 (declared via mix_setrecdevs at :1875). Non-zero values from the mask (32, 64, 96, 128, ..., 992) are used as direct array indices into the 25-element envy24ht_mixmap[], causing OOB reads 128-3968 bytes past the array. KASAN would catch this; production silently returns whatever garbage the OOB read yields, which is then used as a channel index. Any user with /dev/mixer access can trigger this via a MIXER_SETRECSRC ioctl.

Why not live-reproduced on the QEMU guest

The envy24ht sound PCI driver attaches only to ICEnsemble VT1724/Envy24ht hardware, which is absent from the QEMU guest. /dev/mixer does not exist on this guest.

Recommended fix

Convert the bitmask to an index in envy24htmixer_setrecsrc: 'if (src == 0 || (src & (src-1)) != 0 || ffs(src) > NELEM(envy24ht_mixmap)) return src; int ch = envy24ht_mixmap[ffs(src)-1];'.

Kernel references (confirmed during verification)

• sys/dev/sound/pci/envy24ht.c:1956 (ch = envy24ht_mixmap[src] with raw bitmask)
• sys/dev/sound/pci/envy24ht.c:236 (envy24ht_mixmap[] is 25 entries)
• sys/dev/sound/pci/envy24ht.h:172 (ENVY24HT_MIX_REC_MASK = 0x3e0)
• sys/dev/sound/pci/envy24ht.c:1875 (mix_setrecdevs declares the mask)

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