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

[Linus Torvalds]

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call that makes it invalid, so that by the time you exit it is no
longer valid etc.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().

                Linus
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