Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram_poll()

Rainer Weikusat [off-list ref] writes:

Mathias Krause [off-list ref] writes:

quoted

On 2 October 2015 at 22:43, Jason Baron [off-list ref] wrote:

quoted

The unix_dgram_poll() routine calls sock_poll_wait() not only for the wait
queue associated with the socket s that we are poll'ing against, but also calls

[useless full-quote removed]

quoted

My reproducer runs on this patch for more than 3 days now without
triggering anything anymore.

Since the behaviour of your program is random, using it to "test"
anything doesn't really provide any insight: It could have been
executing the same codepath which doesn't happen to trigger any problems
for all of these three days. Nobody can tell.

Since this "strangely" seems to have been lost in the thread: Here's the
test program showing that the reconnect while in epoll actually causes a
problem (at least I think so):

--------
#include <fcntl.h>
#include <pthread.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/epoll.h>
#include <signal.h>
#include <unistd.h>

static int sk, tg0, tg1;

static void *epoller(void *unused)
{
    struct epoll_event epev;
    int epfd;
    
    epfd = epoll_create(1);
    if (epfd == -1) exit(0);

    epev.events = EPOLLOUT;
    epoll_ctl(epfd, EPOLL_CTL_ADD, sk, &epev);
    epoll_wait(epfd, &epev, 1, 5000);

    close(sk);

    execl("./a.out", "./a.out", (void *)0);

    return NULL;
}

int main(void)
{
    struct sockaddr_un sun;
    pthread_t tid;
    int rc;

    sun.sun_family = AF_UNIX;
    
    tg0 = socket(AF_UNIX, SOCK_DGRAM, 0);
    strncpy(sun.sun_path, "/tmp/tg0", sizeof(sun.sun_path));
    unlink(sun.sun_path);
    bind(tg0, (struct sockaddr *)&sun, sizeof(sun));
    
    tg1 = socket(AF_UNIX, SOCK_DGRAM, 0);
    strncpy(sun.sun_path, "/tmp/tg1", sizeof(sun.sun_path));
    unlink(sun.sun_path);
    bind(tg1, (struct sockaddr *)&sun, sizeof(sun));

    sk = socket(AF_UNIX, SOCK_DGRAM, 0);
    connect(sk, (struct sockaddr *)&sun, sizeof(sun));

    fcntl(sk, F_SETFL, fcntl(sk, F_GETFL) | O_NONBLOCK);
    
    while ((rc = write(sk, "bla", 3)) != -1);

    pthread_create(&tid, NULL, epoller, NULL);

    usleep(5);

    strncpy(sun.sun_path, "/tmp/tg0", sizeof(sun.sun_path));
    connect(sk, (struct sockaddr *)&sun, sizeof(sun));
    close(tg1);
    
    pause();

    return 0;
}
----------

And here the other demonstrating the poller not being woken up despite
it could write something:

----------
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/poll.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
    struct sockaddr_un sun;
    struct pollfd pfd;
    int tg, sk0, sk1, rc;
    char buf[16];

    sun.sun_family = AF_UNIX;
    
    tg = socket(AF_UNIX, SOCK_DGRAM, 0);
    strncpy(sun.sun_path, "/tmp/tg", sizeof(sun.sun_path));
    unlink(sun.sun_path);
    bind(tg, (struct sockaddr *)&sun, sizeof(sun));
    
    sk0 = socket(AF_UNIX, SOCK_DGRAM, 0);
    connect(sk0, (struct sockaddr *)&sun, sizeof(sun));
    
    sk1 = socket(AF_UNIX, SOCK_DGRAM, 0);
    connect(sk1, (struct sockaddr *)&sun, sizeof(sun));

    fcntl(sk0, F_SETFL, fcntl(sk0, F_GETFL) | O_NONBLOCK);
    fcntl(sk1, F_SETFL, fcntl(sk1, F_GETFL) | O_NONBLOCK);
    
    while (write(sk0, "bla", 3) != -1);

    if (fork() == 0) {
	pfd.fd = sk1;
	pfd.events = POLLOUT;
	rc = poll(&pfd, 1, -1);

	_exit(0);
    }
    
    sleep(3);
    read(tg, buf, sizeof(buf));
    wait(&rc);

    return 0;
}
-----------