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

[Andrew Lutomirski]

On Tue, Apr 22, 2014 at 9:43 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Apr 22, 2014 at 9:33 AM, Andrew Lutomirski <amluto@xxxxxxxxx> wrote:
>>
>> For the espfix_adjust_stack thing, when can it actually need to do
>> anything?  irqs should be off, I think, and MCE, NMI, and debug
>> exceptions use ist, so that leaves just #SS and #GP, I think.  How can
>> those actually occur?  Is there a way to trigger them deliberately
>> from userspace?  Why do you have three espfix_adjust_stack
>
> Yes, you can very much trigger GP deliberately.
>
> The way to do it is to just make an invalid segment descriptor on the
> iret stack. Or make it a valid 16-bit one, but make it a code segment
> for the stack pointer, or read-only, or whatever. All of which is
> trivial to do with a sigretun system call. But you can do it other
> ways too - enter with a SS that is valid, but do a load_ldt() system
> call that makes it invalid, so that by the time you exit it is no
> longer valid etc.
>
> There's a reason we mark that "iretq" as taking faults with that
>
>         _ASM_EXTABLE(native_iret, bad_iret)
>
> and that "bad_iret" creates a GP fault.
>
> And that's a lot of kernel stack. The whole initial GP fault path,
> which goes to the C code that finds the exception table etc. See
> do_general_protection_fault() and fixup_exception().

My point is that it may be safe to remove the special espfix fixup
from #PF, which is probably the most performance-critical piece here,
aside from iret itself.

--Andy
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