linux-kernel - Re: [PATCH v6 0/4] x86: modify_ldt improvement, test, and config option

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Message-ID: <55BB7FDE.80709@citrix.com>
Date:	Fri, 31 Jul 2015 15:02:06 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	"Sasha Levin" <sasha.levin@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v6 0/4] x86: modify_ldt improvement, test, and config
 option

On 31/07/15 14:44, Boris Ostrovsky wrote:
> On 07/31/2015 05:10 AM, Andrew Cooper wrote:
>> On 30/07/15 22:31, Andy Lutomirski wrote:
>>> This is intended for x86/urgent.  Sorry for taking so long, but it
>>> seemed nice to avoid breaking Xen.
>> Very much appreciated.  Thanks!
>>
>>> This fixes the "dazed and confused" issue which was exposed by the
>>> CVE-2015-5157 fix.  It's also probably a good general attack surface
>>> reduction, and it replaces some scary code with IMO less scary code.
>>>
>>> Also, servers and embedded systems should probably turn off modify_ldt.
>>> This makes that possible.
>>>
>>> Xen people, can you test patch 1?  It works for me on my evil 32-bit
>>> Xen virtio setup.
>> So the LDT issue seems to have gone away, which is good.
>>
>> However, I did get this from my single vcpu guest test
>>
>> [OK]    LDT entry 0 is invalid
>> [SKIP]    Cannot set affinity to CPU 1
>> [RUN]    Test exec
>> [    3.638967] CPU 0 set the LDT
>> [OK]    LDT entry 0 has AR 0x0040FA00 and limit 0x0000002A
>> [    3.639380] ------------[ cut here ]------------
>> [    3.639389] WARNING: CPU: 0 PID: 383 at
>> /local/linux-mainline.git/arch/x86/include/asm/mmu_context.h:96
>> flush_old_exec+0x7fd/0xb70()
>> [    3.639397] DEBUG_LOCKS_WARN_ON(!irqs_disabled())
>
> You must be running v5 (or earlier). This is fixed in v6 --- it is now
> 'DEBUG_LOCKS_WARN_ON(preemptible());'

Hmm - I definitely have the correct code, but did a complete clean and
rebuild, and the issue went away.  I presume I had something stale in
the build.

I am still seeing

[    5.496264] WARNING: CPU: 0 PID: 389 at
/local/linux-mainline.git/kernel/locking/lockdep.c:2639
trace_hardirqs_off_caller+0xa9/0xb0()
[    5.496272] DEBUG_LOCKS_WARN_ON(!irqs_disabled())
[    5.496276] CPU: 0 PID: 389 Comm: ldt_gdt_32 Not tainted 4.2.0-rc4+ #21

But that looks incidental, and unrelated to these fixes.

~Andrew
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