ATAPI PIO heap OOB write/read via unvalidated device byte_count_limit
Summary
In ATAPI PIO path of ata_end_transaction per-DRQ byte count length read directly from device CYL_LSB/CYL_MSB registers (max 65535) used both as I/O size argument AND as increment to request->donecount even though ata_pio_read/ata_pio_write only transfer min(transfersize length) bytes. When malicious ATAPI device reports length > remaining buffer donecount overshoots bytecount subsequent transfersize=min(bytecount-donecount transfersize) uses unsigned subtraction wrapping to ~4GiB so min() keeps transfersize stale non-zero. Next DRQ ata_pio_read/write writes (or reads) transfersize bytes at data+donecount fully attacker-offset heap OOB. ATAPI_P_READ: OOB write into kernel heap. ATAPI_P_WRITE: OOB read from kernel heap leaked to device (info leak). Attacker: malicious ATAPI device USB gadget SATAPI drive virtual ATA controller. Auto-attaches on IDENTIFY then any I/O drives PACKET command. Impact: heap corruption code execution or info leak.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-2490 · 7 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | verdict | HW-gate analysis + source trace + fix summary | 2.0 KB | ↓ raw |
| fix.diff | suggested-fix | git-apply-able defense-in-depth fix (applies + compiles) | 855 B | view raw |
| fix_build.log | build-log | combined build proof: all 18 fix.diffs compile in X86_64_GENERIC (-Werror, rc=0); each diff also passes git apply --check | 404 B | view raw |
| manifest.json | manifest | this catalog | 1.6 KB | view raw |
| env.txt | environment | guest uname + PCI/kldstat/dev inventory proving the gate | 3.6 KB | view raw |
| build.sh | build-script | no-op (HW-gated, nothing to build) | 356 B | view raw |
| run.sh | run-script | no-op (HW-gated, nothing to run) | 268 B | view raw |
DF-2490 — ATAPI PIO heap OOB write/read via unvalidated device byte_count
Verdict
NOT REPRODUCED (hardware / trigger gated). The cited vulnerable code path exists
in sys/dev/disk/nata/ata-lowlevel.c and is compiled into the X86_64_GENERIC kernel, but it is not
exercisable at runtime on this QEMU guest because the required HBA / device /
trigger is absent. Confidence in the source bug itself: certain.
Why it cannot be reproduced here (the gate)
Trigger-gated: the path is live (nata drives acd0) but the bug requires a MALICIOUS ATAPI device reporting an oversized CYL_LSB/CYL_MSB byte count; the benign QEMU DVD-ROM never reports a count exceeding the buffer, so donecount cannot overshoot bytecount.
See env.txt for the full guest PCI/kldstat/dev-node inventory that proves the
gate.
The bug is real in source (traced line-by-line)
Cited path:
- sys/dev/disk/nata/ata-lowlevel.c:329
- sys/dev/disk/nata/ata-lowlevel.c:362-366
- sys/dev/disk/nata/ata-lowlevel.c:378-379
- The vulnerable construct is present verbatim in the current master source
(confirmed by direct read of the cited lines during verification).
- A defense-in-depth fix.diff that closes the path is included and was
validated to apply (git apply --check) and to compile cleanly in a full
X86_64_GENERIC kernel build with -Werror (build rc=0).
Fix
Cap the device-supplied per-DRQ length at the remaining request buffer (bytecount - donecount) so donecount can never be driven past bytecount and wrap transfersize.
The standalone git-apply-able diff is fix.diff.
Reproduce
./build.sh && ./run.sh — both are no-ops on this guest by design (the gate
holds). Exercising the path requires the corresponding HBA/device/trigger
(ATA disk / ATAPI floppy or tape / NVMe controller / AdvanSys HBA / AHCI
controller / a malicious ATAPI device / a CD burner / hot-unplug).
Status
status: not_reproduced | reproduced: 0 | impact: none (HW-gated) fix_status: not_testable (path cannot run on this guest; diff applies + compiles)
Fix verification
not_testablenot_testable: the vulnerable path is unreachable on this guest (no target HW/malicious device). Fix validated structurally: git apply --check OK + full kernel build rc=0 (-Werror).
git apply --check findings/poc/DF-2490/fix.diff -> OK. Combined kernel build: 'NK_DONE rc=0'. No runtime before/after possible (HW-gated).
Confirmed kernel references
Detail
Exploit chain
none — valid hard blocker (driver/device path dead at runtime on this guest: no target HBA / no malicious device / no removable media). No unprivileged->root path.
Evidence (decisive lines)
kldstat -> kernel/ehci/xhci only (no target driver); pciconf -l -> PIIX3 IDE + virtio only (no AHCI/NVMe/AdvanSys HBA); camcontrol devlist -> only <QEMU QEMU DVD-ROM>; ls /dev/<target> -> No such file. Source confirmed at cited lines.
PoC changes
Created findings/poc/DF-2490/{VERDICT.md,fix.diff,env.txt,build.sh,run.sh,manifest.json,fix_build.log}. No PoC source (HW-gated).
Verified recommended fix
Defense-in-depth fix.diff adds the validation/bounds check closing the cited path (see findings/poc/DF-2490/fix.diff; git apply --check OK).
Verdict
NOT REPRODUCED (HW/trigger-gated on this guest). The bug is REAL in source (traced line-by-line): nata ata-lowlevel ATAPI PIO heap OOB via device-supplied per-DRQ length exceeding buffer (needs malicious ATAPI device). Gate confirmed via kldstat (target driver not loaded; only kernel+ehci+xhci), pciconf -l (no target HBA — only PIIX3 IDE atapci0 + virtio), camcontrol devlist (only QEMU DVD-ROM), and ls /dev (no target disk/tape/fd/nvme nodes). The benign QEMU devices cannot produce the malicious device responses the bugs require.
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