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

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Date:	Sat, 25 Jul 2015 09:03:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
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>, 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 Sat, Jul 25, 2015 at 2:15 AM, Borislav Petkov <bp@...en8.de> 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.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/Kconfig                   | 17 +++++++++++++++++
>>  arch/x86/include/asm/mmu.h         |  2 ++
>>  arch/x86/include/asm/mmu_context.h | 31 +++++++++++++++++++++++--------
>>  arch/x86/kernel/Makefile           |  3 ++-
>>  arch/x86/kernel/cpu/perf_event.c   |  4 ++++
>>  arch/x86/kernel/process_64.c       |  2 ++
>>  arch/x86/kernel/step.c             |  2 ++
>>  kernel/sys_ni.c                    |  1 +
>>  8 files changed, 53 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index b3a1a5d77d92..ede52be845db 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -1015,6 +1015,7 @@ config VM86
>>  config X86_16BIT
>>       bool "Enable support for 16-bit segments" if EXPERT
>>       default y
>> +     depends on MODIFY_LDT_SYSCALL
>>       ---help---
>>         This option is required by programs like Wine to run 16-bit
>>         protected mode legacy code on x86 processors.  Disabling
>> @@ -2053,6 +2054,22 @@ config CMDLINE_OVERRIDE
>>         This is used to work around broken boot loaders.  This should
>>         be set to 'N' under normal conditions.
>>
>> +config MODIFY_LDT_SYSCALL
>> +       bool "Enable the LDT (local descriptor table)" if EXPERT
>
>         bool "Enable modify_ldt() for per-process Local Descriptor Table"
>
> is how I'd call it.

Okay with me.

>
>> +       default y
>
> Is that "default y" going to turn into a "default n" after a grace
> period?

Let's see how Willy's default-off sysctl plays out.  In the long run,
maybe we'll have it compiled in but runtime-disabled by default.
There's a big community of users who *really* like using Wine :)

--Andy
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