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SubjectRe: Retbleed, Zen2 and STIBP
On Mon, Jul 18, 2022 at 04:58:49PM +0200, Alexandre Chartre wrote:
> On Mon, Jul 18, 2022 at 09:19:01AM +0100, Ken Moffat wrote:
> > Probably like most people, I find the detail of the available
> > retbleed mitigations obscure. In particular, for zen2 the options
> > *might* include ibpb or unret.
> >
> > But I have failed to find what 'unret' actually means. Any
> > pointers, please ?
> >
> > While ibpb might be available (and slow), on my Renoir with
> > microcode level (0860106h) there were no newer microcode versions
> > available when I last looked (a few weeks ago) but note 7 at the
> > bottom of
> > https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v
> > 7_20220712.pdf
> > implies that the relevant bit is only set on Renoir in 0860109h and
> > later.
> >
> > Some of the text in that pdf implies that at least one of the
> > options could be set if not already set from the microcode, but the
> > amount of detail leaves me totally lost.
> >
> > Assuming, for the moment, that I might want to try this full
> > mitigation, is there any way to set this in the absence of newer
> > microcode ?
> >
> > Or should I just accept that the best I can get is 'unret', whatever
> > that means ?
> >
> > ĸen
>
> 'unret' = AMD JMP2RET i.e. replace all 'ret' instructions with
> 'jmp __x86_return_thunk', and safe training the thunk code upon
> kernel/hypervisor entry. This is a purely software mitigation,
> it doesn't require any microcode.
>
> AMD JMP2RET is described in this document:
> https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v7_20220712.pdf
>
> alex.

Thanks!

ĸen
--
It is very easy to get ridiculously confused about the tenses of
time travel, but most things can be resolved by a sufficiently
large ego. -- The Last Continent

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