Andy Lutomirski <l...@kernel.org> wrote: > On Fri, Feb 16, 2018 at 12:42 AM, Linus Torvalds > <torva...@linux-foundation.org> wrote: >> On Thu, Feb 15, 2018 at 4:22 PM, Nadav Amit <nadav.a...@gmail.com> wrote: >>> It is not too pretty, I agree, but it should do the work. There is only one >>> problematic descriptor that can be used to switch from compatibility-mode to >>> long-mode in the GDT (LDT descriptors always have the L-bit cleared). >>> Changing the descriptor's present bit on context switch when needed can do >>> the work. >> >> Sure, I can see it working, but it's some really shady stuff, and now >> the scheduler needs to save/restore/check one more subtle bit. >> >> And if you get it wrong, things will happily work, except you've now >> defeated PTI. But you'll never notice, because you won't be testing >> for it, and the only people who will are the black hats. >> >> This is exactly the "security depends on it being in sync" thing that >> makes me go "eww" about the whole model. Get one thing wrong, and >> you'll blow all the PTI code out of the water. >> >> So now you tried to optimize one small case that most people won't >> use, but the downside is that you may make all our PTI work (and all >> the overhead for all the _normal_ cases) pointless. > > There's also the fact that, if this stuff goes in, we'll be > encouraging people to deploy 32-bit binaries. Then they'll buy > Meltdown-fixed CPUs (or AMD CPUs!) and they may well continue running > 32-bit binaries. Sigh. I'm not totally a fan of this.
Ok, ok. Stop kicking the dead body...
