linux-kernel - Re: [PATCH 4/5] x86-64: Replace vsyscall gettimeofday fallback with int 0xcc

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Date:	Sun, 29 May 2011 15:57:05 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] x86-64: Replace vsyscall gettimeofday fallback with
 int 0xcc

On Sun, May 29, 2011 at 3:49 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Andrew Lutomirski <luto@....edu> wrote:
>
>> > Ok, i suspect you marked it 0xCC because that's the INT3 instruction
>> > - not very useful for exploits?
>>
>> Exactly.
>>
>> The comments in irq_vectors.h make it sound like vectors 0x81..0xed
>> are used for device interrupts but AFAICT it's only 0x20..0x39 that
>> are used, so the precise choice of vector doesn't matter that much.
>
> No, we use almost all of the vector space for device interrupts. Why
> do you think only 0x20..0x39 is used?

Possibility my inability to understand all the IRQ mapping code in
just half an hour of trying.

In arch/x86/kernel/irq.c, arch_probe_nr_irqs returns NR_IRQS_LEGACY,
which I think means that the genirq code allocates will only expect
IRQs on that many vectors.

If I'm wrong then my patch could be bad: if something tries to use
vector 0xcc for a device interrupt, then the vsyscall emulation code
will eat that interrupt.

(0xcc is barely below the maximum.  INVALIDATE_TLB_VECTOR_START could
be as low as 0xcf.)

--Andy
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