linux-kernel - [PATCH net v4 4/4] vsock/test: add stream TX credit bounds test

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Message-Id: <20251217181206.3681159-5-mlbnkm1@gmail.com>
Date: Wed, 17 Dec 2025 19:12:06 +0100
From: Melbin K Mathew <mlbnkm1@...il.com>
To: stefanha@...hat.com,
	sgarzare@...hat.com
Cc: kvm@...r.kernel.org,
	netdev@...r.kernel.org,
	virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	mst@...hat.com,
	jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	Melbin K Mathew <mlbnkm1@...il.com>
Subject: [PATCH net v4 4/4] vsock/test: add stream TX credit bounds test

Add a regression test for the TX credit bounds fix. The test verifies
that a sender with a small local buffer size cannot queue excessive
data even when the peer advertises a large receive buffer.

The client:
  - Sets a small buffer size (64 KiB)
  - Connects to server (which advertises 2 MiB buffer)
  - Sends in non-blocking mode until EAGAIN
  - Verifies total queued data is bounded

This guards against the original vulnerability where a remote peer
could cause unbounded kernel memory allocation by advertising a large
buffer and reading slowly.

Suggested-by: Stefano Garzarella <sgarzare@...hat.com>
Signed-off-by: Melbin K Mathew <mlbnkm1@...il.com>
---
 tools/testing/vsock/vsock_test.c | 103 +++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)

diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 0e8e173dfbdc..9f4598ee45f9 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -347,6 +347,7 @@ static void test_stream_msg_peek_server(const struct test_opts *opts)
 }
 
 #define SOCK_BUF_SIZE (2 * 1024 * 1024)
+#define SMALL_SOCK_BUF_SIZE (64 * 1024ULL)
 #define MAX_MSG_PAGES 4
 
 static void test_seqpacket_msg_bounds_client(const struct test_opts *opts)
@@ -2203,6 +2204,103 @@ static void test_stream_nolinger_server(const struct test_opts *opts)
 	close(fd);
 }
 
+static void test_stream_tx_credit_bounds_client(const struct test_opts *opts)
+{
+	unsigned long long sock_buf_size;
+	char buf[4096];
+	size_t total = 0;
+	ssize_t sent;
+	int fd;
+	int flags;
+
+	memset(buf, 'A', sizeof(buf));
+
+	fd = vsock_stream_connect(opts->peer_cid, opts->peer_port);
+	if (fd < 0) {
+		perror("connect");
+		exit(EXIT_FAILURE);
+	}
+
+	sock_buf_size = SMALL_SOCK_BUF_SIZE;
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_MAX_SIZE)");
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+
+	flags = fcntl(fd, F_GETFL);
+	if (flags < 0) {
+		perror("fcntl(F_GETFL)");
+		exit(EXIT_FAILURE);
+	}
+
+	if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+		perror("fcntl(F_SETFL)");
+		exit(EXIT_FAILURE);
+	}
+
+	control_expectln("SRVREADY");
+
+	for (;;) {
+		sent = send(fd, buf, sizeof(buf), 0);
+		if (sent > 0) {
+			total += sent;
+			continue;
+		}
+		if (sent < 0 && (errno == EAGAIN || errno == EWOULDBLOCK))
+			break;
+
+		perror("send");
+		exit(EXIT_FAILURE);
+	}
+
+	/*
+	 * With TX credit bounded by local buffer size, sending should
+	 * stall quickly. Allow some overhead but fail if we queued an
+	 * unreasonable amount.
+	 */
+	if (total > (size_t)(SMALL_SOCK_BUF_SIZE * 4)) {
+		fprintf(stderr,
+			"TX credit too large: queued %zu bytes (expected <= %llu)\n",
+			total, (unsigned long long)(SMALL_SOCK_BUF_SIZE * 4));
+		exit(EXIT_FAILURE);
+	}
+
+	control_writeln("CLIDONE");
+	close(fd);
+}
+
+static void test_stream_tx_credit_bounds_server(const struct test_opts *opts)
+{
+	unsigned long long sock_buf_size;
+	int fd;
+
+	fd = vsock_stream_accept(VMADDR_CID_ANY, opts->peer_port, NULL);
+	if (fd < 0) {
+		perror("accept");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Server advertises large buffer; client should still be bounded */
+	sock_buf_size = SOCK_BUF_SIZE;
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_MAX_SIZE)");
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+
+	control_writeln("SRVREADY");
+	control_expectln("CLIDONE");
+
+	close(fd);
+}
+
 static struct test_case test_cases[] = {
 	{
 		.name = "SOCK_STREAM connection reset",
@@ -2382,6 +2480,11 @@ static struct test_case test_cases[] = {
 		.run_client = test_seqpacket_unread_bytes_client,
 		.run_server = test_seqpacket_unread_bytes_server,
 	},
+	{
+		.name = "SOCK_STREAM TX credit bounds",
+		.run_client = test_stream_tx_credit_bounds_client,
+		.run_server = test_stream_tx_credit_bounds_server,
+	},
 	{},
 };
 
-- 
2.34.1