Verify the holder provided valid witnesses and uses SIGHASH_ALL

LDK checks the following:
 * Each input spends an output that is one of P2WPKH, P2WSH, or P2TR.
   These were already checked by LDK when the inputs to be contributed
   were provided.
 * All signatures use the `SIGHASH_ALL` sighash type.
 * P2WPKH and P2TR key path spends are valid (verifies signatures)

NOTE:
 * When checking P2WSH spends, LDK tries to decode 70-72 byte witness
   elements as ECDSA signatures with a sighash flag. If the internal
   DER-decoding fails, then LDK just assumes it wasn't a signature and
   carries with checks. If the element can be decoded as an ECDSA
   signature, the the sighash flag must be `SIGHASH_ALL`.
 * When checking P2TR script-path spends, LDK assumes all elements of
   exactly 65 bytes with the last byte matching any valid sighash flag
   byte are schnorr signatures and checks that the sighash type is
   `SIGHASH_ALL`. If the last byte is not any valid sighash flag, the
   element is assumed not to be a signature and is ignored. Elements of
   64 bytes are not checked because if they were schnorr signatures then
   they would implicitly be `SIGHASH_DEFAULT` which is an alias of
   `SIGHASH_ALL`.

Author: dunxen

lightning/src/ln/channel.rs

@@ -7990,6 +7990,8 @@ where
 				),
 			})
 			.and_then(|signing_session| {
+				signing_session
+					.verify_interactive_tx_signatures(&self.context.secp_ctx, &witnesses)?;
 				signing_session
 					.provide_holder_witnesses(self.context.channel_id, witnesses)
 					.map_err(|err| APIError::APIMisuseError { err })

lightning/src/ln/channelmanager.rs

@@ -5908,19 +5908,32 @@ where
 	/// counterparty's signature(s) the funding transaction will automatically be broadcast via the
 	/// [`BroadcasterInterface`] provided when this `ChannelManager` was constructed.
 	///
-	/// `SIGHASH_ALL` MUST be used for all signatures when providing signatures.
-	///
-	/// <div class="warning">
-	/// WARNING: LDK makes no attempt to prevent the counterparty from using non-standard inputs which
-	///  will prevent the funding transaction from being relayed on the bitcoin network and hence being
-	///  confirmed.
-	/// </div>
+	/// `SIGHASH_ALL` MUST be used for all signatures when providing signatures, otherwise your
+	/// funds can be held hostage!
+	///
+	/// LDK checks the following:
+	///  * Each input spends an output that is one of P2WPKH, P2WSH, or P2TR.
+	///    These were already checked by LDK when the inputs to be contributed were provided.
+	///  * All signatures use the `SIGHASH_ALL` sighash type.
+	///  * P2WPKH and P2TR key path spends are valid (verifies signatures)
+	///
+	/// NOTE:
+	///  * When checking P2WSH spends, LDK tries to decode 70-72 byte witness elements as ECDSA
+	///    signatures with a sighash flag. If the internal DER-decoding fails, then LDK just
+	///    assumes it wasn't a signature and carries with checks. If the element can be decoded
+	///    as an ECDSA signature, the the sighash flag must be `SIGHASH_ALL`.
+	///  * When checking P2TR script-path spends, LDK assumes all elements of exactly 65 bytes
+	///    with the last byte matching any valid sighash flag byte are schnorr signatures and checks
+	///    that the sighash type is `SIGHASH_ALL`. If the last byte is not any valid sighash flag, the
+	///    element is assumed not to be a signature and is ignored. Elements of 64 bytes are not
+	///    checked because if they were schnorr signatures then they would implicitly be `SIGHASH_DEFAULT`
+	///    which is an alias of `SIGHASH_ALL`.
 	///
 	/// Returns [`ChannelUnavailable`] when a channel is not found or an incorrect
 	/// `counterparty_node_id` is provided.
 	///
 	/// Returns [`APIMisuseError`] when a channel is not in a state where it is expecting funding
-	/// signatures.
+	/// signatures or if any of the checks described above fail.
 	///
 	/// [`ChannelUnavailable`]: APIError::ChannelUnavailable
 	/// [`APIMisuseError`]: APIError::APIMisuseError