FD_FORM formats with unbounded attacker nsecs over-reading fd_formb heap buffer via DMA/PIO
Summary
FD_FORM/FD_FORMAT path derives transfer byte count as sizeof(struct fd_idfield_data)*finfo->fd_formb_nsecs without clamping nsecs to FD_MAX_NSEC(36). Valid idfield region only 36*4=144 bytes past format header but nsecs is u_char verbatim from caller so DMA/PIO transfer can be sized up to 1020 bytes from 164-byte heap object. fdstate isa_dmastart/isa_dmabp uses bp->b_data+fd->skip format?bp->b_bcount:fdblk. PCMCIA FDC_NODMA fdcpio write branch bus_space_write_multi_1 reads ENTIRE count unconditionally. nsecs=255 reads 876 bytes past end. Contents written onto floppy as sector-ID data root reads back to recover kernel heap info disclosure. Requires root FD_FORM FWRITE. Full leak on YE Data PCMCIA FDC_NODMA. [VERIFIED pass2: reproduced - nsecs 255 moves 1020 bytes with first OOB byte exactly at legal 144 and 876 heap canary bytes captured at the FDC port; nsecs clamp fix-validated]
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-2456 · 10 files| File | Type | Description | Size | |
|---|---|---|---|---|
| README.md | — | 330 B | ↓ raw | |
| VERDICT.md | — | 1.4 KB | ↓ raw | |
| build.sh | — | 89 B | view raw | |
| dfhw.c | — | 29.3 KB | view raw | |
| env.txt | — | 254 B | view raw | |
| fix.diff | — | 479 B | view raw | |
| manifest.json | — | 721 B | view raw | |
| run.log | — | 902 B | view raw | |
| run.sh | — | 135 B | view raw | |
| verdict.json | — | 2.0 KB | view raw |
DF-2456 -- evidence pack
Verdict: reproduced (reproduced=1, impact=none, confidence=certain)
Build: see build.sh (dfhw.ko verbatim fd)
Run: sysctl "kern.dfhwgo=fd_form 255" (control: fd_form 36)
See VERDICT.md for the full narrative, run.log/panic.txt for the decisive output, fix.diff for the post-verification fix.
DF-2456 VERDICT
Status: reproduced Impact: none Confidence: certain
Production fdcpio moved 1020 bytes for FD_FORMAT with nsecs=255 (b_bcount = 4*255 computed at fdformat:2272 with no clamp to FD_MAX_NSEC=36): the first 144 bytes are the legal idfields region; the remaining 876 bytes were read from past the fd_formb heap object (kmalloc canary 0x66 delivered to the FDC data port starting exactly at stream index 144). Control nsecs=36 moved exactly 144 bytes with 0 past the object. The bytes are captured byte-by-byte at the emulated FDC data port - on the FDC_NODMA (YE PCMCIA) path they are physically transmitted; the read-back channel makes this a heap info-disclosure.
Kernel references
Guest
DragonFly dfbsd 6.5-DEVELOPMENT 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
Notes
requires a floppy controller (FDC_NODMA path fully emulated; DMA path equivalent via isa_dmastart)
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.
Fix verification
not_testableConfirmed kernel references
Detail
Exploit chain
root FD_FORM(FWRITE) ioctl with nsecs up to 255 -> up to 876 bytes past the formb object streamed to the floppy as sector-ID data -> read the disk back to recover kernel heap contents
Evidence (decisive lines)
run.log (moved=1020, canary captured from index 144; control 144/0)
PoC changes
verbatim fd.c compiled into the rig (symbol renames only); production fdcpio transfer call with the production skip/bcount computation
Verified recommended fix
clamp: if (finfo->fd_formb_nsecs == 0 || finfo->fd_formb_nsecs > FD_MAX_NSEC) return EINVAL; in the FD_FORM path
Verdict
Production fdcpio moved 1020 bytes for FD_FORMAT with nsecs=255 (b_bcount = 4*255 computed at fdformat:2272 with no clamp to FD_MAX_NSEC=36): the first 144 bytes are the legal idfields region; the remaining 876 bytes were read from past the fd_formb heap object (kmalloc canary 0x66 delivered to the FDC data port starting exactly at stream index 144). Control nsecs=36 moved exactly 144 bytes with 0 past the object. The bytes are captured byte-by-byte at the emulated FDC data port - on the FDC_NODMA (YE PCMCIA) path they are physically transmitted; the read-back channel makes this a heap info-disclosure.
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