Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

[Andrew Lutomirski]

On Tue, Apr 22, 2014 at 10:29 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 04/22/2014 10:19 AM, Linus Torvalds wrote:
>> On Tue, Apr 22, 2014 at 10:11 AM, Andrew Lutomirski <amluto@xxxxxxxxx> wrote:
>>>
>>>>
>>>> Anyway, if done correctly, this whole espfix should be totally free
>>>> for normal processes, since it should only trigger if SS is a LDT
>>>> entry (bit #2 set in the segment descriptor). So the normal fast-path
>>>> should just have a simple test for that.
>>>
>>> How?  Doesn't something still need to check whether SS is funny before
>>> doing iret?
>>
>> Just test bit #2. Don't do anything else if it's clear, because you
>> should be done. You don't need to do anything special if it's clear,
>> because I don't *think* we have any 16-bit data segments in the GDT on
>> x86-64.
>>
>
> And we don't (neither do we on i386, and we depend on that invariance.)
>
> Hence:
>
>  irq_return:
> +       /*
> +        * Are we returning to the LDT?  Note: in 64-bit mode
> +        * SS:RSP on the exception stack is always valid.
> +        */
> +       testb $4,(SS-RIP)(%rsp)
> +       jnz irq_return_ldt
> +
> +irq_return_iret:
>         INTERRUPT_RETURN
> -       _ASM_EXTABLE(irq_return, bad_iret)
> +       _ASM_EXTABLE(irq_return_iret, bad_iret)
>
>
> That is the whole impact of the IRET path.
>
> If using IST for #GP won't cause trouble (ISTs don't nest, so we need to
> make sure there is absolutely no way we could end up nested) then the
> rest of the fixup code can go away and we kill the common path
> exception-handling overhead; the only new overhead is the IST
> indirection for #GP, which isn't a performance-critical fault (good
> thing, because untangling #GP faults is a major effort.)

I'd be a bit nervous about read_msr_safe and friends.  Also, what
happens if userspace triggers a #GP and the signal stack setup causes
a page fault?

--Andy
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