lightningd: accept OP_RETURN closer_scriptpubkey in simple-close check#9292
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lightningd: accept OP_RETURN closer_scriptpubkey in simple-close check#9292nGoline wants to merge 1 commit into
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A peer's closing_complete may set closer_scriptpubkey to a spec-valid OP_RETURN, but close_tx_check only whitelisted the two stored shutdown scripts, so master rejected our own signed tx and force-closed the channel. Accept a zero-value OP_RETURN output, only when option_simple_close is negotiated. Adds a regression test. Reported-by: 0xaudron Changelog-None: option_simple_close is experimental and unreleased.
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Under
option_simple_close, a peer'sclosing_completemay setcloser_scriptpubkeyto a spec-validOP_RETURN(BOLT2 permits it when the closer treats its output as uneconomical).closingdaccepts and signs the closing tx, but master'sclose_tx_checkvalidated every output against the two stored shutdown scripts only, so theOP_RETURNoutput matched neither, master rejected our own signed tx viachannel_internal_error, and the channel force-closed.This was remotely triggerable by any peer once the feature is negotiated.
The fix is to accept an output that is a valid, zero-value
OP_RETURN, but only whenoption_simple_closeis negotiated and only after theLOCAL-scriptmatch. Our own output is always paid toshutdown_scriptpubkey[LOCAL], so anOP_RETURNmatching neither stored script can only be the peer'scloser_scriptpubkey. Being unspendable and zero-value, accepting it preserves the check's anti-exfiltration property.Kept master-side rather than whitelisting
subd-reportednegotiated scripts + validating in the signer since that would weaken the trusted-state check and require an HSM-protocol change affecting external signers (VLS).Added
lightningd/test/run-close_tx_check.c, which covers the accept path, feature-gating, non-zero-value rejection, and that unknown spendable scripts stay rejected. Verified to fail without the fix.Reported by @0xaudron via the security process.
option_simple_closeis experimental (--experimental-simple-close, off by default) and unreleased, meaning no production exposure.