Re: [patch 1/2] x86_64 page fault NMI-safe

[Mathieu Desnoyers]

* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) wrote:
> On Wed, Jul 14, 2010 at 3:37 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > I think the %rip check should be pretty simple - exactly because there
> > is only a single point where the race is open between that 'mov' and
> > the 'iret'. So it's simpler than the (similar) thing we do for
> > debug/nmi stack fixup for sysenter that has to check a range.
> 
> So this is what I think it might look like, with the %rip in place.
> And I changed the "nmi_stack_ptr" thing to have both the pointer and a
> flag - because it turns out that in the single-instruction race case,
> we actually want the old pointer.
> 
> Totally untested, of course. But _something_ like this might work:
> 
> #
> # Two per-cpu variables: a "are we nested" flag (one byte), and
> # a "if we're nested, what is the %rsp for the nested case".
> #
> # The reason for why we can't just clear the saved-rsp field and
> # use that as the flag is that we actually want to know the saved
> # rsp for the special case of having a nested NMI happen on the
> # final iret of the unnested case.
> #
> nmi:
> 	cmpb $0,%__percpu_seg:nmi_stack_nesting
> 	jne nmi_nested_corrupt_and_return
> 	cmpq $nmi_iret_address,0(%rsp)
> 	je nmi_might_be_nested
> 	# create new stack
> is_unnested_nmi:
> 	# Save some space for nested NMI's. The exception itself
> 	# will never use more space, but it might use less (since
> 	# if will be a kernel-kernel transition). But the nested
> 	# exception will want two save registers and a place to
> 	# save the original CS that it will corrupt
> 	subq $64,%rsp
> 
> 	# copy the five words of stack info. 96 = 64 + stack
> 	# offset of ss.
> 	pushq 96(%rsp)   # ss
> 	pushq 96(%rsp)   # rsp
> 	pushq 96(%rsp)   # eflags
> 	pushq 96(%rsp)   # cs
> 	pushq 96(%rsp)   # rip
> 
> 	# and set the nesting flags
> 	movq %rsp,%__percpu_seg:nmi_stack_ptr
> 	movb $0xff,%__percpu_seg:nmi_stack_nesting
> 
> regular_nmi_code:
> 	...
> 	# regular NMI code goes here, and can take faults,
> 	# because this sequence now has proper nested-nmi
> 	# handling
> 	...
> nmi_exit:
> 	movb $0,%__percpu_seg:nmi_stack_nesting

The first thing that strikes me is that we could be interrupted by a standard
interrupt on top of the iret instruction below. This interrupt handler could in
turn be interrupted by a NMI, so the NMI handler would not know that it is
nested over nmi_iret_address. Maybe we could simply disable interrupts
explicitly at the beginning of the handler, so they get re-enabled by iret below
upon return from nmi ?

Doing that would ensure that only NMIs can interrupt us.

I'll look a bit more at the code and come back with more comments if things come
up.

Thanks,

Mathieu

> nmi_iret_address:
> 	iret
> 
> # The saved rip points to the final NMI iret, after we've cleared
> # nmi_stack_ptr. Check the CS segment to make sure.
> nmi_might_be_nested:
> 	cmpw $__KERNEL_CS,8(%rsp)
> 	jne is_unnested_nmi
> 
> # This is the case when we hit just as we're supposed to do the final
> # iret of a previous nmi.  We run the NMI using the old return address
> # that is still on the stack, rather than copy the new one that is bogus
> # and points to where the nested NMI interrupted the original NMI
> # handler!
> # Easy: just reset the stack pointer to the saved one (this is why
> # we use a separate "valid" flag, so that we can still use the saved
> # stack pointer)
> 	movq %__percpu_seg:nmi_stack_ptr,%rsp
> 	jmp regular_nmi_code
> 
> # This is the actual nested case.  Make sure we fault on iret by setting
> # CS to zero and saving the old CS.  %rax contains the stack pointer to
> # the original code.
> nmi_nested_corrupt_and_return:
> 	pushq %rax
> 	pushq %rdx
> 	movq %__percpu_seg:nmi_stack_ptr,%rax
> 	movq 8(%rax),%rdx	# CS of original NMI
> 	testq %rdx,%rdx		# CS already zero?
> 	je nmi_nested_and_already_corrupted
> 	movq %rdx,40(%rax)	# save old CS away
> 	movq $0,8(%rax)
> nmi_nested_and_already_corrupted:
> 	popq %rdx
> 	popq %rax
> 	popfq
> 	jmp *(%rsp)
> 
> Hmm?
> 
>                Linus

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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