Messages in this thread | | | Date | Mon, 18 Jul 2022 16:35:49 +0100 | From | Ken Moffat <> | Subject | Re: Retbleed, Zen2 and STIBP |
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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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