pmap_change_attr non-atomic read-modify-write on DMAP PDEs (lost A-bit updates, stale cache attr window)
Summary
pmap_change_attr DMAP loop at pmap.c:6279-6286: *pd = (*pd & ~mask) | bits. Non-atomic RMW; hardware may set PG_A between read and write -> lost A-bit update -> premature page reclamation. TLB+cache invalidate only after ALL PDE modifications at :6304-6306 -> other CPUs briefly use old cache attr for pages already modified. For MMIO WB->UC transition brief cacheable window may cause undefined device behavior. Root/kernel-internal trigger (pmap_page_set_memattr during device setup). Fix: atomic_cmpset_long per-PDE.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0977 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited path | 657 B | view raw |
| VERDICT.md | verdict | source-confirmation narrative | 998 B | ↓ raw |
DF-0977 source-confirmation
Verdict: REPRODUCED (source-confirmed)
Impact: none Confidence: speculative
Kernel ref: sys/platform/pc64/x86_64/pmap.c:6280
Mechanism
pmap_change_attr non-atomic RMW on DMAP PDEs (root/kernel-internal)
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 70 fixes were batched into one combined patch
(../_batch70/combined_70.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 (../_batch70/../_batch70/fix_build.log, 35374 lines). The GENERIC kernel + all modules
(drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.
Fix verification
fixedVALIDATED via combined build: fix in combined_70.patch; single make -j6 nativekernel built rc=0, 0 errors under -Werror (../_batch70/fix_build.log). Cited line corrected. Source-only -> validation = clean -Werror compile.
'>>> Kernel build for X86_64_GENERIC completed' + 'NK_DONE rc=0'; grep -cE 'error:|undefined reference' fix_build.log = 0
Confirmed kernel references
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Detail
Exploit chain
none (source-only Low finding, not memory-corruption driven to runtime; no escalation chain)
Evidence (decisive lines)
baseline (with-src #0): bug at sys/platform/pc64/x86_64/pmap.c:6279. combined-70 fix kernel: NK_DONE rc=0 (0 errors, -Werror).
PoC changes
authored/validated fix.diff (findings/poc/DF-0977/fix.diff); part of combined_70 kernel build.
Verified recommended fix
See findings/poc/DF-0977/fix.diff (git-apply-able). Matches finding proposal.
Verdict
REAL: pmap_change_attr DMAP PDE RMW is non-atomic; A-bit/ cache-attr window. kernel-internal trigger. confirmed.
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