On Tue, 29 Apr 2014 06:29:04 -0700 "H. Peter Anvin" <h...@linux.intel.com> wrote:
> 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. Ah, right. SYSCALL does not update RSP. :-( Hm, so anything that can fire up right after a SYSCALL must use IST. It's possible to use an alternative IDT that gets loaded as the first thing in an NMI handler, but this gets incredibly ugly... > > 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. The trouble here is that the official Intel documentation describes how to do this and specifically requests the OS to cope with nested NMIs. Petr T -- 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/
