From 8cf10ba890d25228e37406bf2e68212aed7d2166 Mon Sep 17 00:00:00 2001
From: Max Lv <max.c.lv@gmail.com>
Date: Wed, 14 Oct 2026 21:10:49 +0800
Subject: [PATCH] Do not memcpy from an empty span in the stream decoder

An empty read before the IV arrived passed a null source to memcpy,
which UBSan reports (and the ASan CI job turns into a failure).
---
 src/crypto/stream.cpp            | 4 +++-
 tests/test_crypto_stream_cpp.cpp | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/crypto/stream.cpp b/src/crypto/stream.cpp
index b8dd8db2..fb92c8a3 100644
--- a/src/crypto/stream.cpp
+++ b/src/crypto/stream.cpp
@@ -217,7 +217,9 @@ Result<StreamTcpDecryptor::Step> StreamTcpDecryptor::decrypt(std::span<const std
     if (!ready_) {
         std::size_t want = key_.spec.iv_len - iv_have_;
         std::size_t take = std::min(want, in.size());
-        std::memcpy(iv_.data() + iv_have_, in.data(), take);
+        if (take != 0) { // an empty span may have a null data()
+            std::memcpy(iv_.data() + iv_have_, in.data(), take);
+        }
         iv_have_ += take;
         step.consumed += take;
         in = in.subspan(take);
diff --git a/tests/test_crypto_stream_cpp.cpp b/tests/test_crypto_stream_cpp.cpp
index 1adea04f..5e660c80 100644
--- a/tests/test_crypto_stream_cpp.cpp
+++ b/tests/test_crypto_stream_cpp.cpp
@@ -179,6 +179,9 @@ void replay(const sc::StreamKey &key)
     std::uint8_t out[16];
     sc::StreamTcpDecryptor first, second;
     SS_CHECK(first.init(key, seen.filter()) && second.init(key, seen.filter()));
+    // An empty read before the IV arrives is a no-op.
+    auto none = first.decrypt(std::span<const std::uint8_t>(), out);
+    SS_CHECK(none && none->consumed == 0 && none->produced == 0 && !first.iv_received());
     // The IV alone is enough to register it.
     auto step = first.decrypt(std::span<const std::uint8_t>(wire.data(), key.spec.iv_len), out);
     SS_CHECK(step && step->produced == 0 && first.iv_received());