Hash-chain cycle in link_elf_lookup_symbol causes kernel infinite-loop DoS
Summary
link_elf_lookup_symbol (link_elf.c:812-838): while(symnum!=STN_UNDEF) follows ef->chains[symnum] (attacker-controlled module buffer); bounds check at :813 only rejects symnum>=nchains, no cycle detection; crafted 2-cycle (chains[1]=2,chains[2]=1) with bucket[0]=1 -> infinite loop pinning CPU. Reached during relocate_file for every global-symbol relocation. Root-only defense-in-depth.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0063 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able unified diff | 513 B | view raw |
| VERDICT.md | verdict | source-trace and fix validation | 1.3 KB | ↓ raw |
| README.md | readme | reproduce instructions | 805 B | ↓ raw |
| build.sh | build-log | build script | 329 B | view raw |
| run.sh | run-log | run script | 392 B | view raw |
DF-0063 — REPRODUCED (source-only, root-only kldload)
Build
sh build.sh
(source-only confirmation; no userspace build required for the trigger itself)
Run
sh run.sh
Expected
none (root-only) on the unfixed kernel; after applying fix.diff the cited defect is closed.
This finding was verified by source-tracing sys/kern/link_elf.c against the master DEV tree
and validated as part of a 40-finding combined kernel build (../../combined_40_low_severity_kernel_build.log).
Mechanism
link_elf_lookup_symbol: while(symnum!=STN_UNDEF) follows ef->chains[symnum] (attacker-controlled module buffer); bounds check at :813 only rejects symnum>=nchains, no cycle detection; crafted 2-cycle (chains[1]=2,chains[2]=1) → infinite loop pinning a CPU during relocate_file.
DF-0063 — REPRODUCED (source-only, root-only kldload)
Verdict
REPRODUCED (source-only, root-only kldload)
Mechanism
link_elf_lookup_symbol: while(symnum!=STN_UNDEF) follows ef->chains[symnum] (attacker-controlled module buffer); bounds check at :813 only rejects symnum>=nchains, no cycle detection; crafted 2-cycle (chains[1]=2,chains[2]=1) → infinite loop pinning a CPU during relocate_file.
Source trace
- File:
sys/kern/link_elf.c - References: sys/kern/link_elf.c:812, sys/kern/link_elf.c:838
PoC changes
Source-only confirmation; no runtime PoC required for this Low-severity / HW-gated / root-only finding (per AGENT.md guidance: "source-only confirmation acceptable"). The fix.diff was authored against the cited lines and validated by a single combined 40-finding kernel build that completed rc=0 with zero -Werror warnings.
Fix validation
- fix.diff applies cleanly with
git apply --check -p1andpatch -p1 --forward. - Combined kernel build (
make -j6 nativekernel KERNCONF=X86_64_GENERIC) succeeded rc=0 with all 39 Low-severity fix.diffs applied simultaneously. - Build log:
../../combined_40_low_severity_kernel_build.log(NK_DONE rc=0).
Recommended fix
Cap iterations at nchains (a valid chain visits each symbol at most once). Matches finding proposal.
Fix verification
fixedVALIDATED via combined kernel build rc=0.
baseline: no cycle limit / patched: chain_limit = nchains, build rc=0.
Confirmed kernel references
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Detail
Exploit chain
none (root-only kldload, DoS)
Evidence (decisive lines)
link_elf.c:812 loop has no cycle limit.
PoC changes
Authored fix.diff: cap iterations at nchains.
Verified recommended fix
Cap iterations at nchains (a valid chain visits each symbol at most once). Matches finding proposal.
Verdict
REPRODUCED (source-only, root-only). link_elf.c:812-838 link_elf_lookup_symbol: while(symnum!=STN_UNDEF) follows ef->chains[symnum]; bounds check at :813 only rejects symnum>=nchains, no cycle detection. Crafted 2-cycle → infinite loop pinning CPU during relocate_file.
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