fix: close host key fail-open paths - #259
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Keep accept-new from degrading to unchecked verification when no default known_hosts path can be determined by adding process-lifetime host key pinning. Fail closed when an existing known_hosts path is unreadable or not a regular file, protect check-then-record with an advisory lock file, and make unsupported @cert-authority handling configurable with a reject policy. Validation: CARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo test --lib ssh::tokio_client::host_verification; CARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo check --lib --tests; CARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo clippy --lib --tests -- -D warnings. Closes #242
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type:bug
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Summary
Close the remaining accept-new host key verification fail-open paths by keeping process-local pins when persistent trust state is unavailable, failing closed for unusable known_hosts files, and serializing file-backed TOFU updates with an advisory lock.
What changed
AcceptNewInMemoryso the defaultaccept-newmode no longer falls back to unconditionalNoCheckwhen no default known_hosts path can be determined.@cert-authoritylines configurable withBSSH_CERT_AUTHORITY_POLICY=rejectwhile preserving the existing warn-and-TOFU default.docs/architecture/ssh-client.mdto state the actual host key guarantees.Test plan
CARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo test --lib ssh::tokio_client::host_verificationCARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo check --lib --testsCARGO_TARGET_DIR=/home/inureyes/Development/backend.ai/bssh/target cargo clippy --lib --tests -- -D warningsCloses #242