linux-kernel - Re: [tip:x86/asm] x86/entry/32: Switch INT80 to the new C syscall path

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Date:	Fri, 16 Oct 2015 15:36:00 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [tip:x86/asm] x86/entry/32: Switch INT80 to the new C syscall path

On Fri, Oct 16, 2015 at 2:32 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Fri, Oct 16, 2015 at 11:22 AM, Brian Gerst <brgerst@...il.com> wrote:
>> On Fri, Oct 16, 2015 at 1:34 PM, Borislav Petkov <bp@...en8.de> wrote:
>>> On Fri, Oct 16, 2015 at 08:59:23AM -0700, Andy Lutomirski wrote:
>>>> Wow I am incompetent.
>>>
>>> Bah, it can happen to anyone...
>>>
>>>>     set_system_trap_gate(IA32_SYSCALL_VECTOR, entry_INT80_32);
>>>>
>>>> How did I not catch that in testing?  Can you change that to
>>>> set_system_intr_gate and see if that helps?
>>>
>>> Yeah, that was it. Well spotted, thanks!
>>
>> The INT80 handler doesn't do anything that requires interrupts to be
>> off (it is already on the process stack), so the tracing should be
>> fixed to expect interrupts on. do_int80_syscall_32() can be eliminated
>> too.
>
> Good point.  Then we blow up in potentially interesting ways if an
> iopl-using process does int80 with interrupts off.  Oh well.

The code this replaced ran with interrupts enabled too, so I don't see
this as a regression.  Usermode drivers could already blow up the
system in many different ways.  I doubt there were any that actually
did a system call while interrupts were disabled.

--
Brian Gerst
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