linux-kernel - Re: lost connection to test machine (3)

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Date:   Wed, 27 Dec 2017 19:22:41 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+4396883fa8c4f64e0175@...kaller.appspotmail.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        David Miller <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev <netdev@...r.kernel.org>
Subject: Re: lost connection to test machine (3)

On Wed, Dec 27, 2017 at 7:18 PM, syzbot
<syzbot+4396883fa8c4f64e0175@...kaller.appspotmail.com> wrote:
> Hello,
>
> syzkaller hit the following crash on
> beacbc68ac3e23821a681adb30b45dc55b17488d
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers
>
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: <syzbot+4396883fa8c4f64e0175@...kaller.appspotmail.com>
> It will help syzbot understand when the bug is fixed. See footer for
> details.
> If you forward the report, please keep this part and the footer.

+netfilter maintainers

Here is cleaned reproducer:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <linux/if.h>
#include <linux/netfilter_ipv4/ip_tables.h>

int main()
{
  int fd;

  fd = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
  struct ipt_replace opt = {};
  opt.num_counters = 1;
  opt.size = -1;
  setsockopt(fd, SOL_IP, 0x40, &opt, 0x4);
  return 0;
}


What happens there is that here:

struct xt_table_info *xt_alloc_table_info(unsigned int size)
{
    ...
    if ((SMP_ALIGN(size) >> PAGE_SHIFT) + 2 > totalram_pages)
        return NULL;

size = -1 and SMP_ALIGN(size) = 0, so this still tries to allocate
4GB+delta bytes.

I don't understand why this uses SMP_ALIGN since we add 2 pages on
top, it seems that we could just drop SMP_ALIGN and local SMP_ALIGN
definition altogether.



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