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SubjectRe: 64bit x86: NMI nesting still buggy?
On 04/29/2014 06:05 AM, Jiri Kosina wrote:
>
> We were not able to come up with any other fix than avoiding using IST
> completely on x86_64, and instead going back to stack switching in
> software -- the same way 32bit x86 does.
>

This is not possible, though, because there are several windows during
which if we were to take an exception which doesn't do IST, e.g. NMI, we
are worse than dead -- we are in fact rootable. Right after SYSCALL in
particular.

> So basically, I have two questions:
>
> (1) is the above analysis correct? (if not, why?)
> (2) if it is correct, is there any other option for fix than avoiding
> using IST for exception stack switching, and having kernel do the
> legacy task switching (the same way x86_32 is doing)?

It is not an option, see above.

> [1] http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf
>
> [2] "A special case can occur if an SMI handler nests inside an NMI
> handler and then another NMI occurs. During NMI interrupt
> handling, NMI interrupts are disabled, so normally NMI interrupts
> are serviced and completed with an IRET instruction one at a
> time. When the processor enters SMM while executing an NMI
> handler, the processor saves the SMRAM state save map but does
> not save the attribute to keep NMI interrupts disabled.
> Potentially, an NMI could be latched (while in SMM or upon exit)
> and serviced upon exit of SMM even though the previous NMI
> handler has still not completed."

I believe [2] only applies if there is an IRET executing inside the SMM
handler, which should not normally be the case. It might also have been
addressed since that was written, but I don't know.

-hpa





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