Messages in this thread | | | From | Andrew Cooper <> | Subject | Re: Retbleed, Zen2 and STIBP | Date | Mon, 18 Jul 2022 21:47:15 +0000 |
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On 18/07/2022 09:19, 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.
That's because retbleed is two totally different bugs between Intel and AMD, and on AMD, it's only a subcase.
In this case for AMD, the root bug is called Branch Type Confusion, with Retbleed (and Straight Line Speculation from previous disclosures) being two sub-cases of BTC.
> 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 ?
The microcode doesn't matter. All it does is automatically activate the same bit we set in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d7caac991feeef1b871ee6988fd2c9725df09039
> Or should I just accept that the best I can get is 'unret', whatever > that means ?
"unret" fixes half the problem; the Retbleed subcase specifically. You want IBPB if you want the full fix for Branch Type Confusion.
~Andrew
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