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

Andrew Lutomirski
2014-04-22T17:30:11Z

On Tue, Apr 22, 2014 at 10:26 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Tue, Apr 22, 2014 at 10:11:27AM -0700, H. Peter Anvin wrote:
>> The fastpath interference is:
>>
>> 1. Testing for an LDT SS selector before IRET. This is actually easier
>> than on 32 bits, because on 64 bits the SS:RSP on the stack is always valid.
>>
>> 2. Testing for an RSP inside the espfix region on exception entry, so we
>> can switch back the stack. This has to be done very early in the
>> exception entry since the espfix stack is so small. If NMI and #MC
>> didn't use IST it wouldn't work at all (but neither would SYSCALL).
>>
>> #2 currently saves/restores %rdi, which is also saved further down.
>> This is obviously wasteful.
>
> Btw, can we runtime-patch the fastpath interference chunk the moment we
> see a 16-bit segment? I.e., connect it to the write_idt() path, i.e. in
> the hunk you've removed in there and enable the espfix checks there the
> moment we load a 16-bit segment.
>
> I know, I know, this is not so important right now but let me put it out
> there just the same.

Or we could add a TIF_NEEDS_ESPFIX that gets set once you have a
16-bit LDT entry.

But I think it makes sense to nail down everything else first. I
suspect that a single test-and-branch in the iret path will be lost in
the noise from iret itself.

--Andy