Messages in this thread | | | Date | Tue, 22 Apr 2014 09:43:55 -0700 | Subject | Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE* | From | Linus Torvalds <> |
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On Tue, Apr 22, 2014 at 9:33 AM, Andrew Lutomirski <amluto@gmail.com> 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with a SS that is valid, but do a load_ldt() system call that makes it invalid, so that by the time you exit it is no longer valid etc.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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