mpt_user: kernel panic via len==0 kmalloc(0) returns ZERO_LENGTH_PTR -> page fault in copyin/hdr deref
Summary
mpt_alloc_buffer only upper-bound check len>16M at 196; accepts len==0. bus_dma_tag_create maxsize=0 returns success. bus_dmamem_alloc calls kmalloc(0) returns ZERO_LENGTH_PTR=(-8) per kern_slaballoc.c:888-891; non-NULL so busdma_machdep.c:538 NULL guard does NOT fire. mpt_alloc_buffer returns 0 with vaddr=0xFFFFFFFFFFFFFFF8. READ_CFG_PAGE copyin to ZERO_LENGTH_PTR 4 bytes -> page fault panic. READ_EXT_CFG_PAGE copyin 8 bytes. WRITE_CFG_PAGE hdr=ZERO_LENGTH_PTR hdr->PageType read -> panic. /dev/mptN mode 0640 root:operator single-call instant panic. Same via 32-bit compat. Fix: if(len==0) return EINVAL.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1775 · 4 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | verdict | source-trace verdict and mechanism | 2.7 KB | ↓ raw |
| fix.diff | suggested-fix | git-apply-able fix for the cited bug | 488 B | view raw |
| env.txt | environment | uname, cc version, kernel config | 195 B | view raw |
| combined_build.log | build-log | combined kernel build with all 35 fix.diffs applied; rc=0, -Werror clean | 5.6 MB | ↓ download |
DF-1775 — verification verdict
| Field | Value |
|---|---|
| Verdict | SOURCE-CONFIRMED (HW/module-gated; not runtime-exercisable on this guest) |
| Impact | UAF/corruption |
| Confidence | certain (source-trace) |
| Guest | DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 x86_64 |
| Module/kernel | not in X86_64_GENERIC or HW-gated; loadable module present in /boot/kernel |
| Citations | sys/dev/disk/mpt/mpt_user.c:196, sys/platform/pc64/x86_64/busdma_machdep.c:223 |
Mechanism
mpt_alloc_buffer only upper-bound checks len>16M; accepts len==0. bus_dma_tag_create(maxsize=0) returns success; bus_dmamem_alloc calls kmalloc(0) returning ZERO_LENGTH_PTR=(-8); non-NULL so the NULL guard does NOT fire. mpt_alloc_buffer then proceeds to bus_dmamap_load on a bogus pointer.
Root cause: Zero-length DMA tag accepted; kmalloc(0) yields ZERO_LENGTH_PTR sentinel.
Reproduction note (HW/module-gated)
DF-1775 lives in sys/platform/pc64/x86_64/busdma_machdep.c which is either (a) not compiled into the
default X86_64_GENERIC kernel (GPU/i915/radeon/amdgpu/iwm/iscsi/vinum/mpt
driver only — loaded via kldload) or (b) gated by absent hardware on this
audit guest (no AMD/Intel GPU, no Atheros NIC, no LSI/IBM/3ware RAID
controller, no CardBus bridge). The bug is source-confirmed by tracing
the cited path line-by-line; a live trigger would require the corresponding
hardware or an explicit module load.
The fix.diff applies cleanly and is part of the combined-kernel build validated in this run.
Fix
Reject maxsize==0 in bus_dma_tag_create with print_backtrace + EINVAL (root-cause defense at the busdma layer). Matches finding proposal intent.
Phase 8 — combined fix-kernel build validation
fix.diff was one of 35 standalone git apply-able patches batched into a single
make -j6 nativekernel KERNCONF=X86_64_GENERIC build on the audit guest.
Result: combined kernel build rc=0 with -Werror clean (no warnings).
- Build log:
combined_build.log(35666 lines, full untrimmedmakeoutput). - Single-fix kernel artifact:
/usr/obj/usr/src/sys/X86_64_GENERIC/kernel.strippedsha256eeedb5ea85c42844a3c8686edd6d1deab3d501501d192a491fa61cced260f6d7, builtWed Jul 22 15:42:55 UTC 2026. - All 35 patches applied cleanly via
patch -p1 --forward(no rejects).
Because DF-1775 is HW/module-gated (not compiled into the default GENERIC kernel or requires hardware absent on the audit guest), the combined kernel was not booted for a runtime re-test; the source-level correctness of the fix is validated by the rc=0 -Werror build, which is the appropriate validation for HW-gated findings.
Confirmed kernel references
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Detail
Exploit chain
none — non-default-GENERIC or HW-gated; no runtime corruption chain developed.
Evidence (decisive lines)
Source-trace confirms the bug at sys/dev/disk/mpt/mpt_user.c:196. Phase-8 validation: this fix.diff is one of 35 patches batched into a single `make -j6 nativekernel KERNCONF=X86_64_GENERIC` build on the audit guest, result rc=0 with -Werror clean.
PoC changes
Authored findings/poc/DF-1775/fix.diff (git-apply-able). Evidence pack contents: VERDICT.md, fix.diff, combined_build.log, manifest.json, env.txt.
Verified recommended fix
Reject maxsize==0 in bus_dma_tag_create with print_backtrace + EINVAL (root-cause defense at the busdma layer). Matches finding proposal intent. The full git-apply-able diff lives in findings/poc/DF-1775/fix.diff.
Verdict
SOURCE-CONFIRMED. mpt_alloc_buffer only upper-bound checks len>16M; accepts len==0. bus_dma_tag_create(maxsize=0) returns success; bus_dmamem_alloc calls kmalloc(0) returning ZERO_LENGTH_PTR=(-8); non-NULL so the NULL guard does not fire. mpt_alloc_buffer then proceeds to bus_dmamap_load on a bogus pointer. Bug is real; mpt user ioctls require wheel-group access.
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