DF-1989 PoC

See the parent finding markdown at findings/DF-1989-*.md for the full threat model and PoC steps. This directory is the evidence-pack slot for the PoC runner; the runner will populate it with sources, build.sh / run.sh, full untrimmed logs, env.txt, VERDICT.md, and manifest.json after verification.

DF-1989 — No BAR bounds validation on TPM-reported buffer offsets and sizes

Verdict

REPRODUCED (source-only, HW-gated). The bug is real and the cited data-flow is correct end-to-end, but the audit guest has no TPM CRB device (no /dev/tpm*, no tpmcrb in pciconf -l, no crypto-class PCI device), so the trigger cannot be exercised at runtime. Per the run instructions, source-only confirmation is acceptable for HW-gated findings, so this row is marked status=inconclusive, reproduced=0, impact=none (HW-gated). The recommended fix has been authored, applied to the in-guest source tree, and verified to compile (tpm.ko re-linked with rc=0).

Mechanism (source-traced, every hop cited)

1. Buffer geometry read directly from CRB MMIO registers. sys/dev/crypto/tpm/tpm_crb.c:187-196: c crb_sc->rsp_off = RD8(sc, TPM_CRB_CTRL_RSP_ADDR); /* or RD4+HADDR */ crb_sc->cmd_off = RD4(sc, TPM_CRB_CTRL_CMD_LADDR); crb_sc->cmd_off |= ((uint64_t) RD4(sc, TPM_CRB_CTRL_CMD_HADDR) << 32); crb_sc->cmd_buf_size = RD4(sc, TPM_CRB_CTRL_CMD_SIZE); crb_sc->rsp_buf_size = RD4(sc, TPM_CRB_CTRL_RSP_SIZE); All four values come from device-controlled registers.

2. tpmcrb_fix_buff_offsets only rewrites absolute→relative offsets. sys/dev/crypto/tpm/tpm_crb.c:124-145: c if (crb_sc->cmd_off > base_addr && crb_sc->cmd_off < base_addr + length) crb_sc->cmd_off -= base_addr; if (crb_sc->rsp_off > base_addr && crb_sc->rsp_off < base_addr + length) crb_sc->rsp_off -= base_addr; This performs no bounds check against the BAR size.

3. Only an overlap/size-equality check exists. sys/dev/crypto/tpm/tpm_crb.c:208-219: c if (crb_sc->rsp_off == crb_sc->cmd_off) { if (crb_sc->cmd_buf_size != crb_sc->rsp_buf_size) { ... return ENXIO; } } There is no comparison against rman_get_size(sc->mem_res) anywhere in tpmcrb_attach. (rman_get_size is the canonical BAR-size accessor, sys/sys/rman.h:146.)

4. Unchecked offsets flow into bus_space. sys/dev/crypto/tpm/tpm_crb.c:367-368: bus_write_region_stream_1(sc->mem_res, crb_sc->cmd_off, sc->buf, length); sys/dev/crypto/tpm/tpm_crb.c:396-397: bus_read_region_stream_1(sc->mem_res, crb_sc->rsp_off + TPM_HEADER_SIZE, &sc->buf[TPM_HEADER_SIZE], bytes_available - TPM_HEADER_SIZE); bus_write_region_stream_1 / bus_read_region_stream_1 are bare macros with no bounds gate (they forward straight to bus_space_*_region_stream_1).

5. Reachable without /dev/tpm0 access. sys/dev/crypto/tpm/tpm_crb.c:412-413: c DEVMETHOD(device_shutdown, tpm20_shutdown), DEVMETHOD(device_suspend, tpm20_suspend), both of which route through tpm20_save_state → tpmcrb_transmit. A shutdown or suspend initiated by any user (e.g. init 0, acpiconf -s 3) reaches the unchecked MMIO write.

Why it is not reproduced at runtime on this guest

pciconf -l on the guest shows no tpm or crypto-class device:

vgapci0@pci0:0:2:0: class=0x030000 ... chip=0x11111234 ...   (QEMU stdvga)

(no tpmcrb, no TPM TIS, no /dev/tpm0). tpmcrb_attach is never called, so the vulnerable code path never executes. Reproducing at runtime would require either booting the guest with -device tpm-crb-device against a patched swtpm, or running on hardware with a real CRB TPM that reports malformed register values — neither of which the audit guest provides.

Exploit chain

Not applicable (this is a panic / DoS-class finding on the primary x86 target; bus_space writes past the MMIO mapping fault on unmapped KVA, not into RAM). The finding markdown notes a theoretical I:H variant on platforms where bus_space is direct-mapped to RAM, but DragonFlyBSD's x86 target uses pmap_mapdev, so the practical impact is reliable kernel panic. There is no privilege-escalation primitive to develop.

PoC changes

There is no in-guest runtime PoC possible without a TPM CRB device. I added a trivial placeholder (tpm_crb_oob.c) that documents the trigger and the HW absence, plus build.sh / run.sh. The substantive confirmation is the source trace above.

Recommended fix

Add BAR-bounds validation in tpmcrb_attach, between the existing overlap/size-equality check and the sc->transmit = tpmcrb_transmit assignment. Use rman_get_size(sc->mem_res) (already available via <sys/rman.h> included through tpm20.h) to obtain the BAR size, and reject any configuration where cmd_off + cmd_buf_size or rsp_off + rsp_buf_size exceeds the BAR. The check uses the overflow-safe form off >= bar || size > bar - off to handle off == bar correctly.

The fix in fix.diff matches the structure proposed in the finding markdown and has been verified to compile (see fix_build.log).

Fix validation

The fix was applied to /usr/src in the guest (patch -p1 --forward, PATCH_RC=0), and the tpm KLD module was rebuilt:

cd /usr/src/sys/dev/crypto/tpm
rm -f tpm_crb.o tpm.ko
AWK=awk make KERNCONF=X86_64_GENERIC KMODDIR=/tmp/tpm_test
# rc=0, tpm.ko = 37488 bytes

Full build output is in fix_build.log. The fix compiles cleanly with -Werror. Runtime before/after validation is not_testable because the guest has no TPM CRB device; the patched tpm_crb.c was inspected to confirm the BAR-bounds check is inserted before sc->transmit = tpmcrb_transmit and rejects oversized configurations with ENXIO.

Because the bug is HW-gated on this guest, a runtime before/after kernel boot + PoC re-run is not possible — fix_status = "not_testable", with the diff verified to apply + compile and source-traced to close the path.

References

• sys/dev/crypto/tpm/tpm_crb.c:187-196 — register reads without BAR bounds check
• sys/dev/crypto/tpm/tpm_crb.c:124-145 — tpmcrb_fix_buff_offsets only adjusts, no bounds
• sys/dev/crypto/tpm/tpm_crb.c:208-219 — only overlap/size-equality check; no rman_get_size
• sys/dev/crypto/tpm/tpm_crb.c:367-368,396-397 — unchecked offsets used in bus_space
• sys/dev/crypto/tpm/tpm_crb.c:412-413 — shutdown/suspend reaches transmit
• sys/sys/rman.h:146 — rman_get_size(r) = r_end - r_start + 1