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Re: [PATCH] x86/cet: Clear IST supervisor token busy bits on S3 resume
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Wed, 16 Mar 2022 20:56:52 +0000
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- Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- Thread-topic: [PATCH] x86/cet: Clear IST supervisor token busy bits on S3 resume
On 14/03/2022 14:29, Jan Beulich wrote:
> On 14.03.2022 12:00, Andrew Cooper wrote:
>> Stacks are not freed across S3. Execution just stops, leaving supervisor
>> token busy bits active. Fixing this for the primary shadow stack was done
>> previously, but there is a (rare) risk that an IST token is left busy too.
>> This will manifest as #DF next time the IST vector gets used.
> Thinking about it some more - wouldn't it be more natural to turn off
> CET as CPUs are being brought down (and for the BSP as late as possible
> before actually invoking S3)? That way no new busy bits can be written
> anymore.
I did consider that, but I don't want to go down that route. It's too
easy to let that turn into a bug where we fail to turn shstk back on,
particularly as we have cpu parking rather than full offline.
It's far safer IMO to ensure that shstk is active for all activities in
Xen context.
~Andrew
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