linux-kernel - Re: [PATCH v2 2/3] x86/ldt: Make modify

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Date:	Wed, 22 Jul 2015 14:34:22 +0200
From:	Willy Tarreau <w@....eu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>,
	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 Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

On Tue, Jul 21, 2015 at 11:23:02PM -0700, Andy Lutomirski wrote:
> >> >> +config MODIFY_LDT_SYSCALL
> >> >> +       bool "Enable the LDT (local descriptor table)" if EXPERT
> >> >> +       default y
> >> >> +       ---help---
> >> >> +         Linux can allow user programs to install a per-process x86
> >> >> +        Local Descriptor Table (LDT) using the modify_ldt(2) system
> >> >> +        call.  This is required to run 16-bit or segmented code such as
> >> >> +        DOSEMU or some Wine programs.  It is also used by some very old
> >> >> +        threading libraries.
> >> >> +
> >> >> +        Enabling this feature adds a small amount of overhead to
> >> >> +        context switches and increases the low-level kernel attack
> >> >> +        surface.  Disabling it removes the modify_ldt(2) system call.
> >> >> +
> >> >> +        Saying 'N' here may make sense for embedded or server kernels.
> >> >> +
> >> >
> >> > I believe Wine still uses the LDT for thread-local data, even for 32
> >> > and 64-bit programs.  This is separate from the Linux runtime TLS.
> >> >
> >>
> >> Really?  I thought the whole reason we had three set_thread_area slots
> >> was for Wine.
> >
> > Too bad we have to guess, if only we had the Wine source code under a nicely
> > accessible Git archive or so to check?
> >
> >   git clone git://source.winehq.org/git/wine.git
> >
> > ;-)
> 
> You don't say?
> 
> It appears that Wine uses set_thread_area with a fallback to
> modify_ldt for 32-bit binaries and arch_prctl for 64-bit.

Why wouldn't we have this (as well as X86_16BIT) as a sysctl for the long
term, just like we've finally got rid of NULL mapping ? We would encourage
distros to ship with those settings disabled by default and to only enable
them when breakage is *observed*. Currently I think that adding new config
options will just make distro ship with the option enabled "just in case".

It's also a nice way to discover users of these mechanisms and to suggest
their developers to contemplate other options or to whine loudly.

Willy

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