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Date: Tue, 29 Apr 2014 06:29:04 -0700 From: "H. Peter Anvin" <hpa@...ux.intel.com> To: Jiri Kosina <jkosina@...e.cz>, Steven Rostedt <rostedt@...dmis.org>, Linus Torvalds <torvalds@...ux-foundation.org> CC: linux-kernel@...r.kernel.org, x86@...nel.org, Salman Qazi <sqazi@...gle.com>, Ingo Molnar <mingo@...e.hu>, Michal Hocko <mhocko@...e.cz>, Borislav Petkov <bp@...en8.de>, Vojtech Pavlik <vojtech@...e.cz>, Petr Tesarik <ptesarik@...e.cz>, Petr Mladek <pmladek@...e.cz> Subject: Re: 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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