linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 30 Jul 2015 17:58:20 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 30, 2015 at 5:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jul 30, 2015 at 8:41 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>>
>> On 24/07/2015 23:08, Andy Lutomirski wrote:
>>> user_icebp is set if int $0x01 happens, except it isn't because user
>>> code can't actually do that -- it'll cause #GP instead.
>>>
>>> user_icebp is also set if the user has a bloody in-circuit emulator,
>>> given the name.  But who on Earth has one of those on a system new
>>> enough to run Linux and, even if they have one, why on Earth are they
>>> using it to send SIGTRAP.
>>
>> You do not need either "int $0x01" or an ICE to set user_icebp = 1.  You
>> can use the 0xf1 opcode, which is kinda like 0xcc but generates #DB
>> instead of #BP.
>
> Great.  There's an opcode that invokes an interrupt gate that's not
> marked as allowing unprivileged access, and that opcode doesn't appear
> in the SDM.  It appears in the APM opcode map with no explanation at
> all.
>
> Thanks, CPU vendors.
>
> --Andy

Some Windows programs (running in Wine) use this opcode for
anti-debugging code.  See commit
a1e80fafc9f0742a1776a0490258cb64912411b0.

--
Brian Gerst
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