Subject: Re: [PATCH 14/16] vfs: Implement mount_super_once
To: "Eric W. Biederman" <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>
References: <877ffyzy1j.fsf_-_@x220.int.ebiederm.org>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>, security@debian.org,
        "security@kernel.org" <security@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "security@ubuntu.com >> security" <security@ubuntu.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Willy Tarreau <w@1wt.eu>,
        Aurelien Jarno <aurelien@aurel32.net>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Jann Horn <jann@thejh.net>, Greg KH <greg@kroah.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>, Florian Weimer <fw@deneb.enyo.de>
From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
Message-ID: <57176D1D.70709@gmail.com>
Date: Wed, 20 Apr 2016 07:50:53 -0400
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On 2016-04-19 23:27, Eric W. Biederman wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
>
>> On April 19, 2016 12:25:03 PM PDT, "H. Peter Anvin" <hpa@zytor.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).