Merge bitcoin/bitcoin#26265: POLICY: Relax MIN_STANDARD_TX_NONWITNESS_SIZE to 65 non-witness bytes

b2aa9e85289fc654106a890c35935e9c76c411fb Add release note for MIN_STANDARD_TX_NONWITNESS_SIZE relaxation (Greg Sanders)
8c5b3646b5afe8a61f5c66478d8e11f0d2ce5108 Relax MIN_STANDARD_TX_NONWITNESS_SIZE to 65 non-witness bytes (Greg Sanders)

Pull request description:

  Since the original fix was set to be a "reasonable" transaction to reduce allocations and the true motivation later revealed, it makes sense to relax this check to something more principled.

  There are more exotic transaction patterns that could take advantage of a relaxed requirement, such as 1 input, 1 output OP_RETURN to burn a utxo to fees for CPFP purposes when change isn't practical.

  Two changes could be accomplished:

  1) Anything not 64 bytes could be allowed

  2) Anything above 64 bytes could be allowed

  In the Great Consensus Cleanup, suggestion (2)
  was proposed as a consensus change, and is the simpler of the two suggestions. It would not allow an "empty" OP_RETURN but would reduce the required padding from 22 bytes to 5.

  The functional test is also modified to test the actual case
  we care about: 64 bytes

  Related mailing list discussions here:
  https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-October/020995.html
  And a couple years earlier:
  https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-May/017883.html

ACKs for top commit:
  achow101:
    reACK b2aa9e85289fc654106a890c35935e9c76c411fb
  glozow:
    reACK b2aa9e85289fc654106a890c35935e9c76c411fb
  pablomartin4btc:
    re-ACK https://github.com/bitcoin/bitcoin/commit/b2aa9e85289fc654106a890c35935e9c76c411fb
  jonatack:
    ACK b2aa9e85289fc654106a890c35935e9c76c411fb with some suggestions

Tree-SHA512: c1ec1af9ddcf31b2272209a4f1ee0c5607399f8172e5a1dfd4604cf98bfb933810dd9369a5917ad122add003327c9fcf6ee26995de3aca41d5c42dba527991ad

6 files changed, 63 insertions, 19 deletions

diff --git a/doc/release-notes-26265.md b/doc/release-notes-26265.md
new file mode 100644
index 0000000000..ca2313d956
--- /dev/null
+++ b/doc/release-notes-26265.md
@@ -0,0 +1,6 @@
+P2P and network changes
+---------
+
+- Transactions of non-witness size 65 and above are now allowed by mempool
+  and relay policy. This is to better reflect the actual afforded protections
+  against CVE-2017-12842 and open up additional use-cases of smaller transaction sizes. (#26265)
diff --git a/src/policy/policy.h b/src/policy/policy.h
index 29764ea2d9..8a26e70ef4 100644
--- a/src/policy/policy.h
+++ b/src/policy/policy.h
@@ -25,8 +25,8 @@ static constexpr unsigned int DEFAULT_BLOCK_MAX_WEIGHT{MAX_BLOCK_WEIGHT - 4000};
 static constexpr unsigned int DEFAULT_BLOCK_MIN_TX_FEE{1000};
 /** The maximum weight for transactions we're willing to relay/mine */
 static constexpr unsigned int MAX_STANDARD_TX_WEIGHT{400000};
-/** The minimum non-witness size for transactions we're willing to relay/mine (1 segwit input + 1 P2WPKH output = 82 bytes) */
-static constexpr unsigned int MIN_STANDARD_TX_NONWITNESS_SIZE{82};
+/** The minimum non-witness size for transactions we're willing to relay/mine: one larger than 64  */
+static constexpr unsigned int MIN_STANDARD_TX_NONWITNESS_SIZE{65};
 /** Maximum number of signature check operations in an IsStandard() P2SH script */
 static constexpr unsigned int MAX_P2SH_SIGOPS{15};
 /** The maximum number of sigops we're willing to relay/mine in a single tx */
diff --git a/src/validation.cpp b/src/validation.cpp
index 76bea97341..15d72c903a 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -690,10 +690,7 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return state.Invalid(TxValidationResult::TX_NOT_STANDARD, reason);
     }
 
-    // Do not work on transactions that are too small.
-    // A transaction with 1 segwit input and 1 P2WPHK output has non-witness size of 82 bytes.
-    // Transactions smaller than this are not relayed to mitigate CVE-2017-12842 by not relaying
-    // 64-byte transactions.
+    // Transactions smaller than 65 non-witness bytes are not relayed to mitigate CVE-2017-12842.
     if (::GetSerializeSize(tx, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) < MIN_STANDARD_TX_NONWITNESS_SIZE)
         return state.Invalid(TxValidationResult::TX_NOT_STANDARD, "tx-size-small");
 
diff --git a/test/functional/data/invalid_txs.py b/test/functional/data/invalid_txs.py
index 3747b2a98d..7892410826 100644
--- a/test/functional/data/invalid_txs.py
+++ b/test/functional/data/invalid_txs.py
@@ -46,18 +46,19 @@ from test_framework.script import (
     OP_MOD,
     OP_MUL,
     OP_OR,
+    OP_RETURN,
     OP_RIGHT,
     OP_RSHIFT,
     OP_SUBSTR,
-    OP_TRUE,
     OP_XOR,
 )
 from test_framework.script_util import (
+    MIN_PADDING,
+    MIN_STANDARD_TX_NONWITNESS_SIZE,
     script_to_p2sh_script,
 )
 basic_p2sh = script_to_p2sh_script(CScript([OP_0]))
 
-
 class BadTxTemplate:
     """Allows simple construction of a certain kind of invalid tx. Base class to be subclassed."""
     __metaclass__ = abc.ABCMeta
@@ -122,7 +123,9 @@ class SizeTooSmall(BadTxTemplate):
     def get_tx(self):
         tx = CTransaction()
         tx.vin.append(self.valid_txin)
-        tx.vout.append(CTxOut(0, CScript([OP_TRUE])))
+        tx.vout.append(CTxOut(0, CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 2)))))
+        assert len(tx.serialize_without_witness()) == 64
+        assert MIN_STANDARD_TX_NONWITNESS_SIZE - 1 == 64
         tx.calc_sha256()
         return tx
 
diff --git a/test/functional/mempool_accept.py b/test/functional/mempool_accept.py
index 02ec18140c..7ec3fbe7cf 100755
--- a/test/functional/mempool_accept.py
+++ b/test/functional/mempool_accept.py
@@ -14,7 +14,9 @@ from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     COIN,
     COutPoint,
+    CTransaction,
     CTxIn,
+    CTxInWitness,
     CTxOut,
     MAX_BLOCK_WEIGHT,
     MAX_MONEY,
@@ -26,13 +28,19 @@ from test_framework.script import (
     OP_0,
     OP_HASH160,
     OP_RETURN,
+    OP_TRUE,
 )
 from test_framework.script_util import (
+    DUMMY_MIN_OP_RETURN_SCRIPT,
     keys_to_multisig_script,
+    MIN_PADDING,
+    MIN_STANDARD_TX_NONWITNESS_SIZE,
     script_to_p2sh_script,
+    script_to_p2wsh_script,
 )
 from test_framework.util import (
     assert_equal,
+    assert_greater_than,
     assert_raises_rpc_error,
 )
 from test_framework.wallet import MiniWallet
@@ -333,6 +341,35 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             maxfeerate=0,
         )
 
+        # Prep for tiny-tx tests with wsh(OP_TRUE) output
+        seed_tx = self.wallet.send_to(from_node=node, scriptPubKey=script_to_p2wsh_script(CScript([OP_TRUE])), amount=COIN)
+        self.generate(node, 1)
+
+        self.log.info('A tiny transaction(in non-witness bytes) that is disallowed')
+        tx = CTransaction()
+        tx.vin.append(CTxIn(COutPoint(int(seed_tx[0], 16), seed_tx[1]), b"", SEQUENCE_FINAL))
+        tx.wit.vtxinwit = [CTxInWitness()]
+        tx.wit.vtxinwit[0].scriptWitness.stack = [CScript([OP_TRUE])]
+        tx.vout.append(CTxOut(0, CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 2)))))
+        # Note it's only non-witness size that matters!
+        assert_equal(len(tx.serialize_without_witness()), 64)
+        assert_equal(MIN_STANDARD_TX_NONWITNESS_SIZE - 1, 64)
+        assert_greater_than(len(tx.serialize()), 64)
+
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'tx-size-small'}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
+
+        self.log.info('Minimally-small transaction(in non-witness bytes) that is allowed')
+        tx.vout[0] = CTxOut(COIN - 1000, DUMMY_MIN_OP_RETURN_SCRIPT)
+        assert_equal(len(tx.serialize_without_witness()), MIN_STANDARD_TX_NONWITNESS_SIZE)
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': True, 'vsize': tx.get_vsize(), 'fees': { 'base': Decimal('0.00001000')}}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
 
 if __name__ == '__main__':
     MempoolAcceptanceTest().main()
diff --git a/test/functional/test_framework/script_util.py b/test/functional/test_framework/script_util.py
index b114002145..39dcb418b7 100755
--- a/test/functional/test_framework/script_util.py
+++ b/test/functional/test_framework/script_util.py
@@ -13,12 +13,13 @@ from test_framework.script import (
     OP_EQUAL,
     OP_EQUALVERIFY,
     OP_HASH160,
+    OP_RETURN,
     hash160,
     sha256,
 )
 
 # To prevent a "tx-size-small" policy rule error, a transaction has to have a
-# non-witness size of at least 82 bytes (MIN_STANDARD_TX_NONWITNESS_SIZE in
+# non-witness size of at least 65 bytes (MIN_STANDARD_TX_NONWITNESS_SIZE in
 # src/policy/policy.h). Considering a Tx with the smallest possible single
 # input (blank, empty scriptSig), and with an output omitting the scriptPubKey,
 # we get to a minimum size of 60 bytes:
@@ -28,15 +29,15 @@ from test_framework.script import (
 # Output:      8 [Amount] + 1 [scriptPubKeyLen] = 9 bytes
 #
 # Hence, the scriptPubKey of the single output has to have a size of at
-# least 22 bytes, which corresponds to the size of a P2WPKH scriptPubKey.
-# The following script constant consists of a single push of 21 bytes of 'a':
-#   <PUSH_21> <21-bytes of 'a'>
-# resulting in a 22-byte size. It should be used whenever (small) fake
-# scriptPubKeys are needed, to guarantee that the minimum transaction size is
-# met.
-DUMMY_P2WPKH_SCRIPT = CScript([b'a' * 21])
-DUMMY_2_P2WPKH_SCRIPT = CScript([b'b' * 21])
-
+# least 5 bytes.
+MIN_STANDARD_TX_NONWITNESS_SIZE = 65
+MIN_PADDING = MIN_STANDARD_TX_NONWITNESS_SIZE - 10 - 41 - 9
+assert MIN_PADDING == 5
+
+# This script cannot be spent, allowing dust output values under
+# standardness checks
+DUMMY_MIN_OP_RETURN_SCRIPT = CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 1)))
+assert len(DUMMY_MIN_OP_RETURN_SCRIPT) == MIN_PADDING
 
 def key_to_p2pk_script(key):
     key = check_key(key)