linux-kernel - Re: Bug 4.1.16: self-detected stall in net/unix/?

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Date:	Thu, 11 Feb 2016 18:31:04 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	Philipp Hahn <pmhahn@...ahn.de>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, Karolin Seeger <kseeger@...ba.org>,
	Jason Baron <jbaron@...mai.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arvid Requate <requate@...vention.de>,
	Stefan Gohmann <gohmann@...vention.de>
Subject: Re: Bug 4.1.16: self-detected stall in net/unix/?

Rainer Weikusat <rw@...pelsaurus.mobileactivedefense.com> writes:
> Ben Hutchings <ben@...adent.org.uk> writes:

[...]

>> unix: Fix potential double-unlock in unix_dgram_sendmsg()
>>
>> A datagram socket may be peered with itself, so that sk == other.  We
>> use unix_state_double_lock() to lock sk and other in the right order,
>> which also guards against this and only locks the socket once, but we
>> then end up trying to unlock it twice.  Add the check for sk != other.

[...]

> other was found via
>
> 	if (!other) {
> 		err = -ECONNRESET;
> 		if (sunaddr == NULL)
> 			goto out_free;
>
> 		other = unix_find_other(net, sunaddr, namelen, sk->sk_type,
> 					hash, &err);
> 		if (other == NULL)
> 			goto out_free;
> 	}
>
> and the if-block leading to the double lock should never have been
> executed as it's supposed to deal with the case where sk is connect to
> other but other not to sk (eg, /dev/log).

Test program triggering a double unlock (w/o the MSG_DONTWAIT, it blocks
which it isn't supposed to do):

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

#define SUN_NAME	"\0other-is-me"

int main(int argc, char **argv)
{
    struct sockaddr_un sun;
    unsigned max;
    int sk, rc;

    sk = socket(AF_UNIX, SOCK_DGRAM, 0);

    sun.sun_family = AF_UNIX;
    strncpy(sun.sun_path, SUN_NAME, sizeof(sun.sun_path));
    bind(sk, (struct sockaddr *)&sun, sizeof(sun));

    max = 12;
    do 
	sendto(sk, &sun, sizeof(sun), MSG_DONTWAIT,
	       (struct sockaddr *)&sun, sizeof(sun));
    while (--max);

    return 0;
}