Re: Avoid speculative indirect calls in kernel

[Andrea Arcangeli]

On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
> Thanks for resending it.

Thanks to you for the PTI improvements!

Did my best to do the cleanest patch for tip, but I now figured Dave's
original comment was spot on: a _PAGE_NX clear then becomes necessary
also after pud_alloc not only after p4d_alloc.

pmd_alloc would run into the same with x86 32bit non-PAE too.

So there are two choices, either going back to one single _PAGE_NX
clear from the original Dave's original patch as below, or to add
multiple clear after each level which was my objective and is more
robust, but it may be overkill in this case. As long as it was one
line it looked a clear improvement.

Considering the caller in both cases is going to abort I guess we can
use the one liner approach as Dave and Jiri did originally.

It's up to you, doing it at each level would be more resilent in case
the caller is changed.

For the efi_64 same issue, the current tip patch will work better, but
it can still be cleaned up with pgd_efi instead of pgd_offset_k().

I got partly fooled because it worked great with 4levels, but it
wasn't ok anyway for 32bit non-PAE. Sometime it's the simpler stuff
that gets more subtle.

Andrea