linux-kernel - Re: [PATCH v4 2/3] x86/ldt: Make modify

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Date:	Fri, 24 Jul 2015 23:44:52 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Willy Tarreau <w@....eu>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v4 2/3] x86/ldt: Make modify_ldt optional

On Fri, Jul 24, 2015 at 11:23 PM, Willy Tarreau <w@....eu> wrote:
> On Fri, Jul 24, 2015 at 10:36:45PM -0700, Andy Lutomirski wrote:
>> The modify_ldt syscall exposes a large attack surface and is
>> unnecessary for modern userspace.  Make it optional.
>
> Andy, you didn't respond whether you think it wouldn't be better to make
> it runtime-configurable instead. The goal here is to ensure distros
> ship with modify_ldt disabled by default. But if it means breaking
> compatibility with (rare) existing applications, I'm seeing a risk
> that they'll ship with it enabled instead, which would make the config
> option useless. The CONFIG_DEFAULT_MMAP_ADDR was a good example of
> successful deployment of a hardening measure that has been widely
> adopted despite its (low) risk of breakage in field because it was
> adjustable in field.

I'm all for it, but I think it should be hard-disablable in config,
too, for the -tiny people.  If we add a runtime disable, let's do a
separate patch, and you and Kees can fight over how general it should
be.

>
> That's why here I think we should do the same, and possibly even
> emit a warning once to report the first user of modify_ldt if that
> can help.
>
> What do you think ?

I'm generally in favor.

On the other hand, the current series is already written, might even
be compatible with Xen, and patch 1 at least fixes a real bug.  Maybe
several real bugs.

--Andy
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