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

[David Woodhouse]

On Mon, 2018-01-08 at 02:34 -0800, Paul Turner wrote:
> One detail that is missing is that we still need RSB refill in some
> cases.
> This is not because the retpoline sequence itself will underflow (it
> is actually guaranteed not to, since it consumes only RSB entries
> that it generates.ÂÂ
> But either to avoid poisoning of the RSB entries themselves, or to
> avoid the hardware turning to alternate predictors on RSB underflow.
> 
> Enumerating the cases we care about:
> 
> â 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.

Or IBPB here, yes? That's what we had in the original patch set when
retpoline came last, and what I assume will be put back again once we
*finally* get our act together and reinstate the full set of microcode
patches.

> kernel->kernel independent of SMEP:
> While much harder to coordinate, facilities such as eBPF potentially
> allow exploitable return targets to be created.
> Generally speaking (particularly if eBPF has been disabled) the risk
> is _much_ lower here, since we can only return into kernel execution
> that was already occurring on another thread (which could e.g. likely
> be attacked there directly independent of RSB poisoning.)
> 
> guest->hypervisor, independent of SMEP:
> For guest ring0 -> host ring0 transitions, it is possible that the
> tagging only includes that the entry was only generated in a ring0
> context. Meaning that a guest generated entry may be consumed by the
> host. This admits:

We are also stuffing the RSB on vmexit in the IBRS/IBPB patch set,
aren't we?

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