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SubjectRe: KASAN: user-memory-access Write in n_tty_set_termios
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Date
Hello.

I manually simplified the reproducer. Since the bug is timing dependent,
this reproducer might fail to reproducer the bug. Anyway, I guess that
there is a race condition between

vfree(ldata);
tty->disc_data = NULL;

at n_tty_close() by something (ioctl(TIOCVHANGUP) ?) and

struct n_tty_data *ldata = tty->disc_data;

if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);

at n_tty_set_termios() by ioctl(TCSETS), for the report says that ldata == NULL
("Write of size 512 at addr 0000000000001060").

----------------------------------------
#include <fcntl.h>
#include <linux/futex.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <termios.h>
#define TIOCGPTPEER _IO('T', 0x41)

static const int zero = 0;
static int master_fd = EOF;
static int slave_fd = EOF;
static void execute_call(int call)
{
struct termios term;
char buf[128];
int ptyno = 0;
switch (call) {
case 0:
master_fd = open("/dev/ptmx", O_RDONLY);
ioctl(master_fd, TIOCSPTLCK, &zero);
if (ioctl(master_fd, TIOCGPTN, &ptyno))
break;
sprintf(buf, "/dev/pts/%d", ptyno);
slave_fd = open(buf, O_RDONLY, 0);
usleep(5 * 1000);
ioctl(slave_fd, TIOCVHANGUP, 0);
break;
case 5:
memset(&term, 0, sizeof(term));
ioctl(master_fd, TCSETS, &term);
break;
case 6:
ioctl(master_fd, TIOCGPTPEER, 0);
break;
}
}

struct thread_t {
int created, running, call;
pthread_t th;
};

static struct thread_t threads[16];
static int running;
static int collide;

static void* thr(void* arg)
{
struct thread_t* th = (struct thread_t*)arg;
for (;;) {
while (!__atomic_load_n(&th->running, __ATOMIC_ACQUIRE))
syscall(SYS_futex, &th->running, FUTEX_WAIT, 0, 0);
execute_call(th->call);
__atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
__atomic_store_n(&th->running, 0, __ATOMIC_RELEASE);
syscall(SYS_futex, &th->running, FUTEX_WAKE);
}
return 0;
}

static void execute(int num_calls)
{
int call, thread;
running = 0;
for (call = 0; call < num_calls; call++) {
for (thread = 0; thread < sizeof(threads) / sizeof(threads[0]); thread++) {
struct thread_t* th = &threads[thread];
if (!th->created) {
th->created = 1;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setstacksize(&attr, 128 << 10);
pthread_create(&th->th, &attr, thr, th);
}
if (!__atomic_load_n(&th->running, __ATOMIC_ACQUIRE)) {
th->call = call;
__atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
__atomic_store_n(&th->running, 1, __ATOMIC_RELEASE);
syscall(SYS_futex, &th->running, FUTEX_WAKE);
if (collide && call % 2)
break;
struct timespec ts;
ts.tv_sec = 0;
ts.tv_nsec = 20 * 1000 * 1000;
syscall(SYS_futex, &th->running, FUTEX_WAIT, 1, &ts);
if (running)
usleep((call == num_calls - 1) ? 10000 : 1000);
break;
}
}
}
}

int main(int argc, char *argv[])
{
while (1) {
execute(7);
collide = 1;
execute(7);
}
return 0;
}
----------------------------------------

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