[Devel] [PATCH hci-8.1 v3 2/2] selftests: ve: introduce vtty pair teardown stress test
Eva Kurchatova
eva.kurchatova at virtuozzo.com
Thu Sep 24 18:01:13 MSK 2026
Either end of a vtty pair creates it, and the last close tears it down.
Race condition in older VZKernel vtty implementation allowed both sides
to decide they were the final one and call release_tty() twice, and a
master opening again while a teardown was in flight used to hand out a
reference to a tty_struct about to be freed. Either way, this caused
use-after-free.
Hammer both ends for ownership correctness & thread safety verification.
Sixty seconds by default, enough to bring down a buggy kernel.
https://virtuozzo.atlassian.net/browse/VSTOR-144771
Feature: ve selftest
Signed-off-by: Eva Kurchatova <eva.kurchatova at virtuozzo.com>
---
tools/testing/selftests/ve/.gitignore | 1 +
tools/testing/selftests/ve/Makefile | 1 +
tools/testing/selftests/ve/settings | 1 +
tools/testing/selftests/ve/ve_vtty_stress.c | 319 ++++++++++++++++++++
4 files changed, 322 insertions(+)
create mode 100644 tools/testing/selftests/ve/settings
create mode 100644 tools/testing/selftests/ve/ve_vtty_stress.c
diff --git a/tools/testing/selftests/ve/.gitignore b/tools/testing/selftests/ve/.gitignore
index afa4c568c2c9..9d8d86093827 100644
--- a/tools/testing/selftests/ve/.gitignore
+++ b/tools/testing/selftests/ve/.gitignore
@@ -1,2 +1,3 @@
ve_ns_owner_test
ve_perms_test
+ve_vtty_stress
diff --git a/tools/testing/selftests/ve/Makefile b/tools/testing/selftests/ve/Makefile
index ec40cbc7b3a1..6cd5823c4d93 100644
--- a/tools/testing/selftests/ve/Makefile
+++ b/tools/testing/selftests/ve/Makefile
@@ -4,5 +4,6 @@ CFLAGS += -g -Wall -O2
TEST_GEN_PROGS += ve_ns_owner_test
TEST_GEN_PROGS += ve_perms_test
+TEST_GEN_PROGS += ve_vtty_stress
include ../lib.mk
diff --git a/tools/testing/selftests/ve/settings b/tools/testing/selftests/ve/settings
new file mode 100644
index 000000000000..694d70710ff0
--- /dev/null
+++ b/tools/testing/selftests/ve/settings
@@ -0,0 +1 @@
+timeout=300
diff --git a/tools/testing/selftests/ve/ve_vtty_stress.c b/tools/testing/selftests/ve/ve_vtty_stress.c
new file mode 100644
index 000000000000..4de40a45428b
--- /dev/null
+++ b/tools/testing/selftests/ve/ve_vtty_stress.c
@@ -0,0 +1,319 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * ve_vtty_stress - open and close a vtty pair from both ends at once.
+ *
+ * Either end of a vtty pair creates it, and the last close tears it down.
+ *
+ * Race condition in older VZKernel vtty implementation allowed both sides
+ * to decide they were the final one and call release_tty() twice, and a
+ * master opening again while a teardown was in flight used to hand out a
+ * reference to a tty_struct about to be freed. Either way, this caused
+ * use-after-free.
+ *
+ * Hammer both ends. The master comes from the vzctl ioctl, which does
+ * not look the VE up. The slave is reachable only from inside the VE, as
+ * TTY_MAJOR minor 1..MAX_NR_VTTY_CONSOLES, which vtty_driver() maps to
+ * vttys index minor - 1, so the test builds a VE with cgroup2, sets
+ * ve.veid to the cgroup it made, and runs the slave side there.
+ *
+ * A warning does not stop the host, so the run reads the kernel log too.
+ * One read() of /dev/kmsg is one record, so the test matches vtty driver
+ * splats, as well as generic kernel warning/oops/kasan reports.
+ *
+ * Usage: ve_vtty_stress [seconds] (default 60, or $VE_VTTY_SECS)
+ */
+#define _GNU_SOURCE
+#include <sched.h>
+#include <stdbool.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+#include <linux/major.h>
+#include <sys/sysmacros.h>
+#include <sys/wait.h>
+#include <time.h>
+
+#include "ve_selftest.h"
+#include "../kselftest.h"
+
+/* The ioctl argument; the trailing data[] the uapi struct has is unused. */
+struct vzctl_ve_configure {
+ unsigned int veid, key, val, size;
+};
+
+#define VZCTL_VE_CONFIGURE _IOW('.', 15, struct vzctl_ve_configure)
+#define VE_CONFIGURE_OPEN_TTY 5
+
+#define VTTY_IDX 0 /* the pair both ends work on */
+#define SLAVE_MINOR (VTTY_IDX + 1) /* vtty_driver(): index = minor - 1 */
+#define DEFAULT_SECS 60
+
+/*
+ * Expected open errno codes: the pair may be closing, or gone already,
+ * when an open lands on it.
+ * Anything else is an environment issue or a bug.
+ */
+static bool expected_errno(int err)
+{
+ return err == EBUSY || err == EIO || err == ENXIO;
+}
+
+static int open_master(int ctl_fd, unsigned int veid)
+{
+ struct vzctl_ve_configure c = {
+ .veid = veid,
+ .key = VE_CONFIGURE_OPEN_TTY,
+ .val = VTTY_IDX,
+ };
+
+ return ioctl(ctl_fd, VZCTL_VE_CONFIGURE, &c);
+}
+
+/* The slave end, inside the VE. Returns the errno that stopped it, or 0. */
+static int hammer_slave(const char *node, time_t deadline,
+ unsigned long *opened)
+{
+ int err = 0;
+
+ while (time(NULL) < deadline) {
+ int fd = open(node, O_RDWR | O_NOCTTY | O_NONBLOCK);
+
+ if (fd >= 0) {
+ (*opened)++;
+ close(fd);
+ } else if (!expected_errno(errno)) {
+ err = errno;
+ break;
+ }
+ }
+
+ return err;
+}
+
+/* The kernel log, since a warning fails nothing by itself. */
+static int kmsg_open(void)
+{
+ int fd = open("/dev/kmsg", O_RDONLY | O_NONBLOCK);
+
+ if (fd >= 0)
+ lseek(fd, 0, SEEK_END);
+ return fd;
+}
+
+/*
+ * Check for kernel log warnings, one read() per record, hence
+ * the function name is not checked.
+ */
+static int kmsg_check(int fd)
+{
+ char buf[8192];
+ int found = 0;
+ ssize_t r;
+
+ if (fd < 0)
+ return 0;
+
+ for (;;) {
+ r = read(fd, buf, sizeof(buf) - 1);
+ if (r < 0) {
+ /* Ring overrun, continue with the oldest existing record */
+ if (errno == EPIPE)
+ continue;
+ break;
+ }
+ if (r == 0)
+ break;
+ buf[r] = '\0';
+ /* Known symptoms of a buggy kernel */
+ if (strstr(buf, "vtty") && (strstr(buf, "tty->count") ||
+ strstr(buf, "bad slave count"))) {
+ ksft_print_msg("vtty mismatched count log: %s", buf);
+ found++;
+ }
+ if (strstr(buf, "WARNING: ") || strstr(buf, "Oops: ") ||
+ strstr(buf, ";BUG: ") || strstr(buf, "soft lockup - ")) {
+ ksft_print_msg("kernel log: %s", buf);
+ found++;
+ }
+ }
+ return found;
+}
+
+int main(int argc, char **argv)
+{
+ unsigned long master_opened = 0, slave_opened = 0;
+ int cg, ctid, ctl, kmsg, ready[2], count[2], status;
+ int master_err = 0, slave_err;
+ const char *env = getenv("VE_VTTY_SECS");
+ char path[PATH_MAX], val[16], c;
+ char dir[] = "/tmp/ve_vtty_XXXXXX";
+ char node[sizeof(dir) + 6];
+ time_t deadline;
+ long secs;
+ pid_t pid;
+
+ ksft_print_header();
+ secs = argc > 1 ? atol(argv[1]) : (env ? atol(env) : DEFAULT_SECS);
+ if (secs <= 0)
+ secs = DEFAULT_SECS;
+
+ ctl = open("/dev/vzctl", O_RDWR);
+ if (ctl < 0)
+ ksft_exit_skip("cannot open /dev/vzctl: %s, the vz modules "
+ "(vzdev, vzmon) are needed\n",
+ strerror(errno));
+
+ cg = mount_cg2_fd();
+ if (cg < 0)
+ ksft_exit_skip("cannot mount cgroup2: %s\n", strerror(errno));
+ write_file_at(cg, "cgroup.subtree_control", VE_CONTROLLERS);
+
+ ctid = make_ve(cg, CTID_MIN);
+ if (ctid < 0)
+ ksft_exit_skip("cannot make a VE cgroup\n");
+
+ /* The ve.veid must match between master / slave */
+ snprintf(path, sizeof(path), "%d/ve.veid", ctid);
+ snprintf(val, sizeof(val), "%d", ctid);
+ if (write_file_at(cg, path, val) < 0) {
+ destroy_ve(cg, ctid);
+ ksft_exit_skip("cannot set ve.veid: %s\n", strerror(errno));
+ }
+
+ /* Do not report a failure where the vtty pair cannot be had at all */
+ status = open_master(ctl, ctid);
+ if (status < 0 && errno != EBUSY) {
+ destroy_ve(cg, ctid);
+ ksft_exit_skip("no vtty master: %s\n", strerror(errno));
+ }
+ if (status >= 0)
+ close(status);
+
+ /*
+ * Use temp directory for the node the slave end opens.
+ * Cleaned up by the master side instead of relying on the slave.
+ */
+ if (!mkdtemp(dir)) {
+ destroy_ve(cg, ctid);
+ ksft_exit_fail_msg("mkdtemp: %s\n", strerror(errno));
+ }
+ snprintf(node, sizeof(node), "%s/slave", dir);
+ if (mknod(node, S_IFCHR | 0600, makedev(TTY_MAJOR, SLAVE_MINOR)) < 0) {
+ status = errno;
+ rmdir(dir);
+ destroy_ve(cg, ctid);
+ ksft_exit_fail_msg("mknod: %s\n", strerror(status));
+ }
+
+ ksft_set_plan(1);
+ kmsg = kmsg_open();
+ if (kmsg < 0)
+ ksft_print_msg("cannot read /dev/kmsg: %s, only a crash will "
+ "be noticed\n",
+ strerror(errno));
+
+ if (pipe(ready) || pipe(count)) {
+ unlink(node);
+ rmdir(dir);
+ destroy_ve(cg, ctid);
+ ksft_exit_fail_msg("pipe: %s\n", strerror(errno));
+ }
+
+ deadline = time(NULL) + secs;
+ ksft_print_msg("VE %d, pair %d, %ld seconds\n", ctid, VTTY_IDX, secs);
+
+ pid = fork();
+ if (pid == 0) {
+ close(ready[0]);
+ close(count[0]);
+ if (enter_cgroup(cg, ctid) < 0)
+ _exit(errno ? errno : 1);
+ /* CLONE_NEWVE puts the task in the VE of the cgroup */
+ if (unshare(CLONE_NEWVE | CLONE_NEWPID | CLONE_NEWNS) < 0)
+ _exit(errno ? errno : 1);
+ if (fork() == 0) {
+ unsigned long n = 0;
+ int err;
+
+ /*
+ * Hammering on after the run has given up would hold
+ * the cgroup open against destroy_ve().
+ */
+ prctl(PR_SET_PDEATHSIG, SIGKILL);
+
+ c = 'r';
+ if (write(ready[1], &c, 1) != 1)
+ _exit(errno);
+ err = hammer_slave(node, deadline, &n);
+ /* Send the count either way: it says how far it got. */
+ if (write(count[1], &n, sizeof(n)) != sizeof(n))
+ _exit(errno);
+ _exit(err);
+ }
+ wait(&status);
+ _exit(WIFEXITED(status) ? WEXITSTATUS(status) : 1);
+ } else if (pid < 0) {
+ unlink(node);
+ rmdir(dir);
+ destroy_ve(cg, ctid);
+ ksft_exit_fail_msg("fork: %s\n", strerror(errno));
+ }
+ close(ready[1]);
+ close(count[1]);
+
+ if (read(ready[0], &c, 1) != 1) {
+ waitpid(pid, &status, 0);
+ unlink(node);
+ rmdir(dir);
+ destroy_ve(cg, ctid);
+ ksft_exit_fail_msg("the VE side never came up\n");
+ }
+
+ while (time(NULL) < deadline) {
+ int fd = open_master(ctl, ctid);
+
+ if (fd >= 0) {
+ master_opened++;
+ close(fd);
+ } else if (!expected_errno(errno)) {
+ master_err = errno;
+ break;
+ }
+ }
+
+ if (master_err) {
+ /* The other end runs to its own deadline, stop it */
+ kill(pid, SIGKILL);
+ waitpid(pid, &status, 0);
+ slave_err = 0;
+ } else {
+ if (read(count[0], &slave_opened, sizeof(slave_opened)) !=
+ sizeof(slave_opened))
+ slave_opened = 0;
+ waitpid(pid, &status, 0);
+ slave_err = WIFEXITED(status) ? WEXITSTATUS(status) : EINTR;
+ }
+
+ ksft_print_msg("%lu master opens, %lu slave opens\n", master_opened,
+ slave_opened);
+
+ status = kmsg_check(kmsg);
+ unlink(node);
+ rmdir(dir);
+ destroy_ve(cg, ctid);
+
+ if (master_err)
+ ksft_exit_fail_msg("the master end stopped: %s\n",
+ strerror(master_err));
+ if (slave_err)
+ ksft_exit_fail_msg("the slave end stopped: %s\n",
+ strerror(slave_err));
+ if (!master_opened)
+ ksft_exit_fail_msg("no master was ever opened\n");
+ if (!slave_opened)
+ ksft_exit_fail_msg("no slave was ever opened\n");
+ if (status)
+ ksft_exit_fail_msg("kernel complained %d time(s)\n", status);
+
+ ksft_test_result_pass("vtty pair closed from both ends\n");
+ ksft_finished();
+}
--
2.55.0
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