Re: Avoid speculative indirect calls in kernel

[Andi Kleen]

Hi Linus,

On Wed, Jan 03, 2018 at 03:51:35PM -0800, Linus Torvalds wrote:
> On Wed, Jan 3, 2018 at 3:09 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> > This is a fix for Variant 2 in
> > https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html
> >
> > Any speculative indirect calls in the kernel can be tricked
> > to execute any kernel code, which may allow side channel
> > attacks that can leak arbitrary kernel data.
> 
> Why is this all done without any configuration options?

I was thinking of a config option, but I was struggling with a name.

CONFIG_INSECURE_KERNEL, CONFIG_LEAK_MEMORY? 

And should it be positive or negative?

So I opted to be secure uncontionally.

It would be simple to add however, all hooks are either in the Makefile
or in asm/jump-asm.h

>  - these workarounds should have a way to disable them.
> 

There will be soon patches to add other ways and they have a way
to patch out most of the retpoline overhead at runtime
(basically replace the trampoline with a pure ret)

We just wanted to get the retpoline code out first because
it's the most basic and widest applicable fix.

-Andi