On Tue, Apr 22, 2014 at 9:43 AM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Tue, Apr 22, 2014 at 9:33 AM, Andrew Lutomirski <aml...@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().
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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
