DF-4044 — amdparsemsg MSG_DISCONNECT writes through NULL active_srb after an unexpected reselection

File: sys/dev/disk/amd/amd.c Severity: Medium | Class: CWE-476 NULL Pointer Dereference Status: hardware-gated — source-certain; runtime reproduction impossible on the audit guest (no AMD53C974 HBA — see env.txt). Verdict: inconclusive, confidence certain.

Build & run (on the DragonFly guest)

./build.sh     # compiles the demonstrator (verbatim-copy of the driver walks)
./run.sh       # reproduces the exact branch selection / fault

Expected: the DF-4044 section of run.log shows the panic-branch selection / fault. Full guest details in env.txt.

Fix validation

fix.diff applied (with all DF-4036..DF-4044 patches together) to a clean guest /usr/src copy; make nativekernel KERNCONF=X86_64_GENERIC completed with 0 errors under -Werror (see fix_build.log — amd.o recompiled into the GENERIC kernel). Sources restored to pristine afterwards (md5-verified).

DF-4044 VERDICT — amdparsemsg MSG_DISCONNECT writes through NULL active_srb after an unexpected reselection

Bottom line

Source-certain NULL write: on an unexpected reselection (untagged_srbs[t][l]==NULL and disc_count==0, amd.c:1834-1836) amd_Reselect leaves amd->active_srb NULL after queueing the ABORT (:1843 skipped). A hostile target that ignores ATN and clocks a MSG_DISCONNECT byte in MSG IN reaches amdparsemsg case MSG_DISCONNECT (amd.c:1217) which writes amd->active_srb->SRBState (:1218) — a NULL-page write fault in amd_intr context => kernel panic. The Q_TAG case guards active_srb!=NULL (:1229) and amdhandlemsgreject guards srb!=NULL (:1475); the DISCONNECT arm is the unguarded hole. Verified with a child process taking SIGSEGV on the exact store (run.log DF-4044). Hostile-target gated -> runtime inconclusive on this guest, source certain.

Exploit chain

hostile target: reselect an idle target/lun (active_srb=NULL, driver queues ABORT) -> send MSG_DISCONNECT byte -> amd_MsgInPhase0:1188 -> amdparsemsg:1218 NULL write => panic

Why not reproduced on the guest

The audit guest has no AMD53c974-compatible SCSI HBA (env.txt); amd(4) is compiled into the GENERIC kernel (X86_64_GENERIC:88 device amd) but never attaches, so the hostile-target message flows cannot be exercised at runtime. The exact walks were compiled from the driver source (verbatim amdsentmsg copy / exact branch ladder / exact store) and driven with the real driver states (run.log); the fix is compile-validated (fix_build.log).

Negative results recorded alongside (pass-2 depth)

Kills proven in run.log: msgin_buf[6] per-byte walk max index 4 (bounded); SRB_array[msgin_buf[1]] 0..255 fits SRB_array[256] exactly; msgout shift bcopy fits msgout_buf[6]; amdtimeout/callout family is #if 0 dead code (no timeout-vs-intr race); EEPROM word walks fit eepromBuf[128] exactly; XPT_RESET_BUS ACBFlag race can only strand CCBs (hang, logged, not filed).

Fix

NULL-guard active_srb in the MSG_DISCONNECT arm (skip the state store and disc_count bump; still complete the message). Applied and compile-validated on the guest (fix_build.log); sys/ tree never modified by the audit.

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.