netdev - Re: [RFC PATCH] af_unix: fix entry locking in unix_dgram

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 01 Dec 2015 17:02:33 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] af_unix: fix entry locking in unix_dgram_recvmsg

Rainer Weikusat <rw@...pelsaurus.mobileactivedefense.com> writes:

[...]

> Insofar I understand the comment in this code block correctly,
>
>         err = mutex_lock_interruptible(&u->readlock);
>         if (unlikely(err)) {
>                 /* recvmsg() in non blocking mode is supposed to return -EAGAIN
>                  * sk_rcvtimeo is not honored by mutex_lock_interruptible()
>                  */
>                 err = noblock ? -EAGAIN : -ERESTARTSYS;
>                 goto out;
>         }
>
> setting a receive timeout for an AF_UNIX datagram socket also doesn't
> work as intended because of this: In case of n readers with the same
> timeout, the nth reader will end up blocking n times the timeout.

Test program which confirms this. It starts four concurrent reads on the
same socket with a receive timeout of 3s. This means the whole program
should take a little more than 3s to execute as each read should time
out at about the same time. But it takes 12s instead as the reads
pile up on the readlock mutex and each then gets its own timeout once it
could enter the receive loop.

-------
#include <stdio.h>
#include <sys/signal.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <unistd.h>

#define SERVER_ADDR	"\0multi-timeout"
#define RCV_TIMEO	3

static void set_rcv_timeo(int sk)
{
    struct timeval tv;

    tv.tv_sec = RCV_TIMEO;
    tv.tv_usec = 0;
    setsockopt(sk, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
}

int main(void)
{
    struct sockaddr_un sun;
    struct timeval tv_start, tv_end;
    int sk, dummy;
    
    sun.sun_family = AF_UNIX;
    memcpy(sun.sun_path, SERVER_ADDR, sizeof(SERVER_ADDR));
    sk = socket(AF_UNIX, SOCK_DGRAM, 0);
    bind(sk, (struct sockaddr *)&sun, sizeof(sun));
    set_rcv_timeo(sk);

    gettimeofday(&tv_start, NULL);
    
    if (fork() == 0) {
	read(sk, &dummy, sizeof(dummy));
	_exit(0);
    }
    
    if (fork() == 0) {
	read(sk, &dummy, sizeof(dummy));
	_exit(0);
    }
    
    if (fork() == 0) {
	read(sk, &dummy, sizeof(dummy));
	_exit(0);
    }

    read(sk, &dummy, sizeof(dummy));
    
    while (waitpid(-1, NULL, 0) > 0);

    gettimeofday(&tv_end, NULL);
    printf("Waited for %u timeouts\n",
	   (unsigned)((tv_end.tv_sec - tv_start.tv_sec) / RCV_TIMEO));
    
    return 0;
}
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html