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 -- 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/
