On Thu, Apr 24, 2014 at 3:24 PM, H. Peter Anvin <h...@linux.intel.com> wrote: > On 04/23/2014 09:53 PM, Andrew Lutomirski wrote: >>> >>> - The user can put arbitrary data in registers before returning to the >>> LDT in order to get it saved at a known address accessible from the >>> kernel. With SMAP and KASLR this might otherwise be difficult. >> >> For one thing, this only matters on Haswell. Otherwise the user can >> put arbitrary data in userspace. >> >> On Haswell, the HPET fixmap is currently a much simpler vector that >> can do much the same thing, as long as you're willing to wait for the >> HPET counter to contain some particular value. I have patches that >> will fix that as a side effect. >> >> Would it pay to randomize the location of the espfix area? Another >> somewhat silly idea is to add some random offset to the CPU number mod >> NR_CPUS so that at attacker won't know which ministack is which. > > Since we store the espfix stack location explicitly, as long as the > scrambling happens in the initialization code that's fine. However, we > don't want to reduce locality lest we massively blow up the memory > requirements.
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. >> 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? --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/
