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

[Brian Gerst]

On Tue, Apr 22, 2014 at 1:46 PM, Andrew Lutomirski <amluto@xxxxxxxxx> wrote:
> 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

Maybe make the #GP handler check what the previous stack was at the start:
1) If we came from userspace, switch to the top of the process stack.
2) If the previous stack was not the espfix stack, switch back to that stack.
3) Switch to the top of the process stack (espfix case)

This leaves the IST available for any recursive faults.
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