Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in kernel

[Paul Turner]

On Mon, Jan 8, 2018 at 2:38 AM, Jiri Kosina <jikos@xxxxxxxxxx> wrote:
> On Mon, 8 Jan 2018, Paul Turner wrote:
>
>> user->kernel in the absence of SMEP:
>> In the absence of SMEP, we must worry about user-generated RSB entries
>> being consumable by kernel execution.
>> Generally speaking, for synchronous execution this will not occur (e.g.
>> syscall, interrupt), however, one important case remains.
>> When we context switch between two threads, we should flush the RSB so that
>> execution generated from the unbalanced return path on the thread that we
>> just scheduled into, cannot consume RSB entries potentially installed by
>> the prior thread.
>
> I am still unclear whether this closes it completely, as when HT is on,
> the RSB is shared between the threads, right? Therefore one thread can
> poision it for the other without even context switch happening.
>

See 2.6.1.1 [Replicated resources]:
  "The return stack predictor is replicated to improve branch
prediction of return instructions"

(This is part of the reason that the sequence is attractive; its use
of the RSB to control prediction naturally prevents cross-sibling
attack.)

> --
> Jiri Kosina
> SUSE Labs
>