Re: [PATCH 0/7] IBRS patch series

[David Woodhouse]

On Thu, 2018-01-04 at 11:00 -0800, Linus Torvalds wrote:
> On Thu, Jan 4, 2018 at 9:56 AM, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> wrote:
> > 
> > 
> > Speculation on Skylake and later requires these patches ("dynamic
> > IBRS")
> > be used instead of retpoline[1].
> Can somebody explain this part?
> 
> I was assuming that retpoline would work around this issue on all
> uarchs.
> 
> This seems to say "retpoline does nothing on Skylake+"

On Skylake the target for a 'ret' instruction may also come from the
BTB. So if you ever let the RSB (which remembers where the 'call's came
from get empty, you end up vulnerable.

Other than the obvious call stack of more than 16 calls in depth,
there's also a big list of other things which can empty the RSB,
including an SMI.

Which basically makes retpoline on Skylake+ *very* hard to use
reliably. The plan is to use IBRS there and not retpoline.

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