Messages in this thread | | | Date | Mon, 18 Jul 2022 16:58:49 +0200 | From | Alexandre Chartre <> | Subject | Re: Retbleed, Zen2 and STIBP |
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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.
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