Re: [tip:x86/pti] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

[David Woodhouse]

On Wed, 2018-01-31 at 08:03 +0100, Dominik Brodowski wrote:
> Whether a process needs protection by IBPB on context switches is a
> different question to whether a process should be allowed to be dumped,
> though the former may be a superset of the latter. Enable IBPB on all
> context switches to a different userspace process, until we have a clear
> mitigation strategy for userspace against Spectre-v2 designed and
> implemented.
>
> ...
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂif (tsk && tsk->mm &&
> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ tsk->mm->context.ctx_id != last_ctx_id &&
> -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ get_dumpable(tsk->mm) != SUID_DUMP_USER)
> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ tsk->mm->context.ctx_id != last_ctx_id)
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂindirect_branch_prediction_barrier();


I understand your argument and I sympathise.

But that's going to hurt a *lot*, and we don't even have a viable
proof-of-concept for a userââuser Spectre v2 attack, do we? It's only
theoretical?

Show a working PoC and it makes the argument somewhat more
convincing...

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