On Thu, Apr 24, 2014 at 3:37 PM, H. Peter Anvin <h...@zytor.com> wrote: > On 04/24/2014 03:31 PM, Andrew Lutomirski wrote: >> >> I was imagining just randomizing a couple of high bits so the whole >> espfix area moves as a unit. >> >>> We could XOR with a random constant with no penalty at all. Only >>> problem is that this happens early, so the entropy system is not yet >>> available. Fine if we have RDRAND, but... >> >> How many people have SMAP and not RDRAND? I think this is a complete >> nonissue for non-SMAP systems. >> > > Most likely none, unless some "clever" virtualizer turns off RDRAND out > of spite. > >>>> Peter, is this idea completely nuts? The only exceptions that can >>>> happen there are NMI, MCE, #DB, #SS, and #GP. The first four use IST, >>>> so they won't double-fault. >>> >>> It is completely nuts, but sometimes completely nuts is actually useful. >>> It is more complexity, to be sure, but it doesn't seem completely out >>> of the realm of reason, and avoids having to unwind the ministack except >>> in the normally-fatal #DF handler. #DFs are documented as not >>> recoverable, but we might be able to do something here. >>> >>> The only real disadvantage I see is the need for more bookkeeping >>> metadata. Basically the bitmask in espfix_64.c now needs to turn into >>> an array, plus we need a second percpu variable. Given that if >>> CONFIG_NR_CPUS=8192 the array has 128 entries I think we can survive that. >> >> Doing something in #DF needs percpu data? What am I missing? > > You need the second percpu variable in the espfix setup code so you have > both the write address and the target rsp (read address). >
Duh. :) --Andy -- 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/
