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

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Date:	Sat, 25 Jul 2015 08:23:43 +0200
From:	Willy Tarreau <w@....eu>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	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,
	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 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.

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 ?

Willy

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