stage1: exec stage2 from a sealed memfd, config on stdin#13
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Replace the write-to-/tmp-then-exec path with an in-memory, pathless launch: - Load the payload into an anonymous memfd, seal it (F_SEAL_WRITE plus SHRINK/GROW/SEAL), and execveat() it directly. The bytes measured into PCR 14 are now immutable and are exactly what runs; there is no named, writable inode to swap between measurement and exec, which closes the measure/exec TOCTOU. MFD_EXEC is requested where supported, with an EINVAL fallback for pre-6.3 kernels. - Deliver the config (the raw user-data JSON) on stdin from a second memfd instead of /tmp/stage2-config.json. stdin needs no extra-fd convention (which trips up runtimes like Bun/Node single-file executables) and, being an in-memory file, has no pipe-size limit. No stage2 artifact is left on any named path. example-stage2 now reads its config from stdin; README documents the contract. Config stays unmeasured by design (code-only PCR 14). Verified: full chain boots and powers off in both sha256 and ed25519 modes, with the config JSON received on stdin. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Follows the boot-path security review. Removes the last on-disk artifacts from the stage2 launch and tightens the "measured == executed" invariant.
What changes
Sealed, pathless exec. The payload is loaded into an anonymous
memfd, sealed withF_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_SEAL, and launched withexecveat(fd, "", AT_EMPTY_PATH). Previously we wrote it to/tmp/stage2.exeand exec'd that path, so the measured bytes and the executed bytes were two different objects with a window in between. Now the thing measured into PCR 14, the thing sealed immutable, and the thing executed are one kernel object.MFD_EXECis requested where supported (Linux 6.3+ hardened kernels default new memfds to non-executable), with anEINVALfallback for older kernels.Config on stdin. The raw user-data JSON is delivered on stdin from a second
memfd(dup2'd to fd 0), replacing/tmp/stage2-config.json. stdin is a universal channel that needs no extra-fd convention (which trips up runtimes like Bun/Node single-file executables), and as an in-memory file it has no pipe-size limit / no deadlock. No stage2 artifact is left on any named path.example-stage2now reads its config from stdin; the README documents the new contract. Config remains unmeasured by design (PCR 14 is code-only), consistent with the existing measurement model.Verification
Full chain (
stage0 -> UKI -> stage1 -> example-stage2) boots and powers off cleanly in both sha256 and ed25519 modes; the payload runs from the sealed memfd (argv[0]=stage2) and prints the config JSON received on stdin. Workspace tests pass; no new warnings.🤖 Generated with Claude Code