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SubjectRE: [patch 00/38] x86/retbleed: Call depth tracking mitigation
From: Thomas Gleixner
> Sent: 17 July 2022 16:07
> On Sun, Jul 17 2022 at 09:45, David Laight wrote:
> > From: Thomas Gleixner
> >>
> >> 3) Utilize the retbleed return thunk mechanism by making the jump
> >> target run-time configurable. Add the accounting counterpart and
> >> stuff RSB on underflow in that alternate implementation.
> >
> > What happens to indirect calls?
> > The above would imply that they miss the function entry thunk, but
> > get the return one.
> > Won't this lead to mis-counting of the RSB?
> That's accounted in the indirect call thunk. This mitigation requires
> retpolines enabled.

Thanks, that wasn't in the summary.

> > I also thought that retpolines would trash the return stack?
> No. They prevent that the CPU misspeculates an indirect call due to a
> mistrained BTB.
> > Using a single retpoline thunk would pretty much ensure that
> > they are never correctly predicted from the BTB, but it only
> > gives a single BTB entry that needs 'setting up' to get mis-
> > prediction.
> BTB != RSB

I was thinking about what happens after the RSB has underflowed.
Which is when (I presume) the BTB based speculation happens.

> The intra function call in the retpoline is of course adding a RSB entry
> which points to the speculation trap, but that gets popped immediately
> after that by the return which goes to the called function.

I'm remembering the 'active' instructions in a retpoline being 'push; ret'.
Which is an RSB imbalance.

> > I'm also sure I managed to infer from a document of instruction
> > timings and architectures that some x86 cpu actually used the BTB
> > for normal conditional jumps?
> That's relevant to the problem at hand in which way?

The next problem :-)


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