Re: Avoid speculative indirect calls in kernel

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 04/01/2018 23:47, Tom Lendacky wrote:
> On 1/4/2018 2:05 PM, David Woodhouse wrote:
>> On Thu, 2018-01-04 at 14:00 -0600, Tom Lendacky wrote:
>>> Yes, lfence is sufficient.  As long as the target is in the register
>>> before the lfence and we jump through the register all is good, i.e.:
>> Thanks. Can I have a Reviewed-by: for this then please:
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
>
> While this works, a more efficient way to do the lfence support would be
> to not use the retpoline in this case.  Changing the indirect jumps to
> do the "mov [rax], rax; lfence; jmp *rax" sequence would be quicker. I'm
> not sure if this is feasible given the need to do a retpoline if you can't
> use lfence, though.

That would be most efficient for AMD, but it isn't compatible with
having a single binary which can mitigate itself most efficiently
wherever it was booted.  On most hardware, we'll want to dynamically
chose between repoline and lfence depending on vendor.

One option would be to teach GCC/Clang/Other to output alternative
patch-point data for indirect branches in the format Linux/Xen could
consume, and feed this into the alternatives framework.

The practical option to actually deploy in the timeframe is to use
__x86.indirect_thunk.%reg and alternate between repoline and lfence in
15 locations, which does add an unconditional call/jmp over the most
efficient alternative, but allows us to switch the thunk-in-use at boot
time.

~Andrew