linux-kernel - Re: [PATCH 14/16] vfs: Implement mount_super

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Date:	Wed, 20 Apr 2016 07:50:53 -0400
From:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH 14/16] vfs: Implement mount_super_once

On 2016-04-19 23:27, Eric W. Biederman wrote:
> "H. Peter Anvin" <hpa@...or.com> writes:
>
>> On April 19, 2016 12:25:03 PM PDT, "H. Peter Anvin" <hpa@...or.com> wrote:
>>>
>>> Perhaps a (privileged) option to exempt from the global limit, then.
>>> Something we can implement if asked for.
>>>
>>> However, I wouldn't be 100% that the reserved pool isn't used.  Someone
>>> added it presumably for a reason.  An administrator could say it and
>>> we'd have no idea.
>>
>> ... and if I personally was running a container-hosting system, I
>> would *absolutely* set it to make sure the administrator could not get
>> locked out.
>
> That is likely easier done by setting:
> echo RIDICULOUSLY_LARGE_NUMBER > /proc/sys/kernel/pty/max
This may protect against administrative lockout on a sane system with 
responsible users, but it doesn't protect you from lockout due to a DoS 
attack, while the reserved pool does (or at least, it makes sure you can 
still allocate a few PTY's even when under attack).