[LTP] [PATCH v4 3/4] tst_kconfig: add module mappings for vsock/vsock_loopback

[Li Wang <li.wang@linux.dev>]

From: "Li Wang (Hieco)" <li.wang@linux.dev>

Map CONFIG_VSOCKETS to "vsock" and CONFIG_VSOCKETS_LOOPBACK to
"vsock_loopback" so tst_kconfig can load them on demand.

cve-2025-21756 binds to VMADDR_CID_LOCAL and therefore needs
vsock_loopback. Without the module loaded, bind() fails with
EADDRNOTAVAIL and the test aborts with TBROK. Declare both configs in
needs_kconfigs so the modules are loaded first.

The test needs root only to load the modules; the reproducer runs as
an ordinary non-root process in the initial namespace, where it can
use loopback VSOCK on non-privileged ports. Drop privileges to nobody
after the modules are loaded, and regain root in cleanup() so the
modules can be unloaded again.

Signed-off-by: Li Wang (Hieco) <li.wang@linux.dev>
---
 lib/tst_kconfig.c              |  2 ++
 testcases/cve/cve-2025-21756.c | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/lib/tst_kconfig.c b/lib/tst_kconfig.c
index 421852ba5..dc025c9f0 100644
--- a/lib/tst_kconfig.c
+++ b/lib/tst_kconfig.c
@@ -174,6 +174,8 @@ static struct config_module_map {
 	{"CONFIG_NET_ACT_PEDIT", "act_pedit"},
 	{"CONFIG_PKCS8_PRIVATE_KEY_PARSER", "pkcs8_key_parser"},
 	{"CONFIG_X509_CERTIFICATE_PARSER", "x509_key_parser"},
+	{"CONFIG_VSOCKETS", "vsock"},
+	{"CONFIG_VSOCKETS_LOOPBACK", "vsock_loopback"},
 	{}
 };
 
diff --git a/testcases/cve/cve-2025-21756.c b/testcases/cve/cve-2025-21756.c
index 4245a4c95..70dddabb7 100644
--- a/testcases/cve/cve-2025-21756.c
+++ b/testcases/cve/cve-2025-21756.c
@@ -13,12 +13,38 @@
  * Beware, this test will crash the system.
  */
 
+#include <pwd.h>
+
 #include "tst_test.h"
 #include "lapi/vm_sockets.h"
 
 #define MAX_PORT_RETRIES	24
 #define VMADDR_CID_NONEXISTING	42
 
+static void drop_privileges(void)
+{
+	struct passwd *pw = SAFE_GETPWNAM("nobody");
+
+	SAFE_SETEGID(pw->pw_gid);
+	SAFE_SETEUID(pw->pw_uid);
+}
+
+static void restore_privileges(void)
+{
+	SAFE_SETEGID(0);
+	SAFE_SETEUID(0);
+}
+
+static void setup(void)
+{
+	drop_privileges();
+}
+
+static void cleanup(void)
+{
+	restore_privileges();
+}
+
 static int vsock_bind(unsigned int cid, unsigned int port, int type)
 {
 	int sock;
@@ -85,8 +111,16 @@ static void run(void)
 }
 
 static struct tst_test test = {
+	.needs_root = 1,
 	.test_all = run,
+	.setup = setup,
+	.cleanup = cleanup,
 	.taint_check = TST_TAINT_W | TST_TAINT_D,
+	.needs_kconfigs = (const char *const[]) {
+		"CONFIG_VSOCKETS",
+		"CONFIG_VSOCKETS_LOOPBACK",
+		NULL
+	},
 	.tags = (const struct tst_tag[]) {
 		{"linux-git", "fcdd2242c0231032fc84e1404315c245ae56322a"},
 		{"CVE", "2025-21756"},
-- 
2.56.0


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