openssh-unix-dev May 2011 archive
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openssh-unix-dev: Re: port-linux.c bug with oom_adjust_restore()

Re: port-linux.c bug with oom_adjust_restore() - causes real bad oom_adj - which can cause DoS conditions.

From: Cal Leeming [Simplicity Media Ltd] <cal.leeming_at_nospam>
Date: Tue May 31 2011 - 11:26:52 GMT
To: Gert Doering <gert@greenie.muc.de>

Bug report submitted to Debian:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=628690

Should be interesting to see what happens with this lol.

On Tue, May 31, 2011 at 12:11 PM, Cal Leeming [Simplicity Media Ltd]
<cal.leeming@simplicitymedialtd.co.uk> wrote:
> Hi Gert,
>
> Thank you for taking the time to respond! Further comments below:
>
> On Tue, May 31, 2011 at 8:25 AM, Gert Doering <gert@greenie.muc.de> wrote:
>> Hi,
>>
>> On Mon, May 30, 2011 at 10:32:24PM +0100, Cal Leeming [Simplicity Media Ltd] wrote:
>>> So, it turns out that it is actually OpenSSH which is broken, after
>>
>> I would not second this. To me, this very much looks like:
>>
>>> On 30/05/2011 21:56, Cal Leeming [Simplicity Media Ltd] wrote:
>>> > Just did some testing..
>>> >
>>> >root@vicky:~# cat /var/log/auth.log | grep "Set"
>>> >May 30 21:41:05 vicky sshd[1568]: Set /proc/self/oom_adj from -17 to -17
>>> >May 30 21:41:07 vicky sshd[1574]: Set /proc/self/oom_adj to -17
>>
>> ... it's reading out the old value, saving it, setting it to "-17" (for
>> the sshd listener process, that one is not to be killed), and later on
>> *restoring* the old value (for all child processes). See the comments
>> in platform.c
>>
>> The log messages look weird because the value is -17 already when sshd
>> starts - so it's adjusting "-17 to -17" and later on "restoring -17" -
>> looks stupid, but that's computers for you. But what it tells you is
>> that the value isn't set by sshd to "-17" but that sshd inherited that
>> from whoever started it.
>
> Could you point out the line of code where oom_adj_save is set to the
> original value, because I've looked everywhere, and from what I can
> tell, it's only ever set to INT_MIN, and no where else is it changed.
> (C is not my strongest language tho, so I most likely have overlooked
> something). This is where I got thrown off.
>
>>
>> The question here is why sshd is sometimes started with -17 and sometimes
>> with 0 - that's the bug, not that sshd keeps what it's given.
>>
>> (Ask yourself: if sshd had no idea about oom_adj at all, would this make
>> it buggy by not changing the value?)
>
> This was what I was trying to pinpoint down before. I had came to this
> conclusion myself, sent it to the Debian bug list, and they dismissed
> on the grounds that it was an openssh problem...
>
> So far, the buck has been passed from kernel-mm to debian-kernel, to
> openssh, and now back to debian-kernel lol. The most annoying thing,
> is that you can't get this bug to happen unless you physically test on
> a machine which requires the bnx2 firmwire, so I get the feeling this
> won't get resolved :/
>
>>
>>
>> Anyway, as a workaround for your system, you can certainly set
>>
>> oom_adj_save = 0;
>>
>> in the beginning of port-linux.c / oom_adjust_restore(), to claim that
>> "hey, this was the saved value to start with" and "restore" oom_adj to 0
>> then - but that's just hiding the bug, not fixing it.
>
> I'm disappointed this wasn't the correct fix, I honestly thought I had
> patched it right :(
>
> But, on the other hand, ssh users should really never have a default
> oom_adj of -17, so maybe 0 should be set as default anyway? If this is
> not the case, could you give reasons why??
>
>>
>> gert
>> --
>> USENET is *not* the non-clickable part of WWW!
>> //www.muc.de/~gert/
>> Gert Doering - Munich, Germany gert@greenie.muc.de
>> fax: +49-89-35655025 gert@net.informatik.tu-muenchen.de
>>
>
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