X-Debbugs-CC: ard.biesheu...@linaro.org On Sun, Aug 21, 2016 at 02:46:06PM +0100, Ben Hutchings wrote: > > > I seem to remember that AArch64 has the ill-advised rule that VA bits > > > outside the range of the current page table format are ignored, so > > > presumably you're concerned that the code relies on this. But since > > > other 64-bit architectures (at least x86, PowerPC and SPARC) behave > > > otherwise, I would expect semi-portable code to mask out the tag bits.
OK, as per my reply to Ian's comment, I now understand your point here. If this is true, then that does reduce the scope of the problem. > > You're not wrong, but unfortunately the ability to write semi-portable > > code left the planet over a decade ago. For clarification - the > > problem is not with regards to code written specifically for arm64 and > > not verified with different MMU-configurations, but with code written > > for x86 and never tested on anything else. See my post to cross-distro > > last week (and the subsequent thread): > > https://lists.linaro.org/pipermail/cross-distro/2016-August/000838.html > > I already read that. I don't see the contradiction. > > > So simply adjusting the mmap range on Debian would result in us using > > less verified code paths without it helping with the problem at hand. > > What new code path would be used? Looking at the code, apparently none other than the one that sets /proc/sys/vm/mmap_rnd_bits, so fair enough. But this still scares me, as it still relies on software that has been proven to not be sane being sane. I would be much less nervous with an approach like the one suggested by Ard at: https://patchwork.kernel.org/patch/9292139/ But then maybe I'm just being too nervous. / Leif