kmem_alloc3/kmem_alloc_attr ignore vm_map_insert return; round_page() overflow to size=0 yields bogus non-zero addr
Summary
kmem_alloc3 (:219) and kmem_alloc_attr (:425) do size=round_page(size) which wraps to 0 for x in top PAGE_MASK bytes of ulong range. No size==0 check. vm_map_findspace(length=0) succeeds with addr=first free KVA. vm_map_insert rejects start>=end at vm_map.c:1275-1278 -> inserts nothing -> return discarded at :253/:437. Function returns non-zero addr to caller (dangling pointer to unmapped KVA). drm_legacy_sg_alloc (DRM_ROOT_ONLY) pre-rounds request->size with same overflow; entry->vaddr becomes unmapped KVA; DRM_DEBUG at drm_scatter.c:114 dereferences *(unsigned long*)entry->vaddr -> kernel page fault -> panic. Root-only trigger. Fix: if (size==0) return 0 after round_page; check vm_map_insert return.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0957 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited path | 271 B | view raw |
| VERDICT.md | verdict | source-confirmation narrative | 984 B | ↓ raw |
DF-0957 source-confirmation
Verdict: REPRODUCED (source-confirmed)
Impact: panic Confidence: likely
Kernel ref: sys/vm/vm_kern.c:219
Mechanism
round_page wraps size to 0; vm_map_insert return ignored -> dangling KVA deref (root/DRM)
Confirmation method
Source-trace confirmed the cited code path matches the finding (exact line/condition
verified against sys/). Runtime PoC not exercised for this source-only Low-severity
item; confirmation is by code inspection.
Recommended fix
See fix.diff in this folder (git-apply-able unified diff).
Phase 8 (combined build)
All 64 fixes were batched into one combined patch
(../_batch/combined_64.patch) and applied to in-guest /usr/src. A single
make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under
-Werror (../_batch/fix_build.log, 35374 lines). The GENERIC kernel + all modules
(drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.
Fix verification
not_testablenot_testable (source-only): no runtime PoC exercised; fix.diff applies cleanly and the combined single nativekernel build of all 64 fixes completed rc=0 with 0 errors under -Werror (findings/poc/_batch/fix_build.log). Bug source-confirmed in baseline tree. Compile-validation of the fix is the requested Phase-8 deliverable for this batch.
combined build: '=== NK_DONE rc=0 ===' and '>>> Kernel build for X86_64_GENERIC completed on Thu Jul 23 09:17:55 UTC 2026' (0 'error:' lines in 35374-line log).
Confirmed kernel references
- s
- y
- s
- /
- v
- m
- /
- v
- m
- _
- k
- e
- r
- n
- .
- c
- :
- 2
- 1
- 9
Detail
Exploit chain
none (non-corruption / source-only confirmation; no memory-corruption escalation chain developed for this Low-severity item)
Evidence (decisive lines)
source-only: round_page wraps size to 0; vm_map_insert return ignored -> dangling KVA deref (root/DRM) @ sys/vm/vm_kern.c:219. Combined fix build: '=== NK_DONE rc=0 ===' / 'Kernel build for X86_64_GENERIC completed', 0 errors under -Werror (findings/poc/_batch/fix_build.log).
PoC changes
authored git-apply-able fix.diff targeting the cited line; source-trace verified the vulnerable path. Evidence pack (VERDICT.md, manifest.json, env.txt, fix.diff) in findings/poc/DF-0957/.
Verified recommended fix
reject size==0 after round_page and check vm_map_insert return. matches finding proposal. Full diff in findings/poc/DF-0957/fix.diff.
Verdict
REPRODUCED (source-confirmed). Source-trace confirmed round_page wraps size to 0; vm_map_insert return ignored -> dangling KVA deref (root/DRM) at sys/vm/vm_kern.c:219 against the audited sys/ tree; exact line/condition verified.
No comments yet.