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Re: [PATCH] x86/SVM: restrict hardware SSBD update upon guest VIRT_SPEC_CTRL write
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Fri, 9 Dec 2022 10:35:20 +0000
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- Thread-topic: [PATCH] x86/SVM: restrict hardware SSBD update upon guest VIRT_SPEC_CTRL write
On 09/12/2022 09:59, Roger Pau Monné wrote:
> On Thu, Dec 08, 2022 at 12:24:54PM +0100, Jan Beulich wrote:
>> core_set_legacy_ssbd() counts the number of times SSBD is being enabled
>> via LS_CFG on a core. This assumes that calls there only occur if the
>> state actually changes. While svm_ctxt_switch_{to,from}() conform to
>> this, guest_wrmsr() doesn't: It also calls the function when the bit
>> doesn't actually change. Extend the conditional there accordingly.
>>
>> Fixes: b2030e6730a2 ("amd/virt_ssbd: set SSBD at vCPU context switch")
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> This is the less intrusive but more fragile variant of a fix. The
>> alternative would be to have core_set_legacy_ssbd() record per-thread
>> state, such that the necessary checking can be done there.
> Hm, yes, it's going to take a bit more of memory to keep track of
> this.
It shouldn't. Turn the count field into a bitmap of thread_ids.
The interface to this functionality should be WRMSR-like, and should
support repeated writes of the same value. Anything else is a timebomb
which has already gone off once...
I'll have a play with this while looking into the repro I've got.
~Andrew
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