Re: 64bit x86: NMI nesting still buggy?

[H. Peter Anvin]

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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