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Re: Revert of the 4.17 hypercall handler changes Re: [PATCH-for-4.17] xen: fix generated code for calling hypercall handlers
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: George Dunlap <George.Dunlap@xxxxxxxxxx>
- Date: Fri, 4 Nov 2022 21:04:55 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Henry Wang <Henry.Wang@xxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: Revert of the 4.17 hypercall handler changes Re: [PATCH-for-4.17] xen: fix generated code for calling hypercall handlers
> On 4 Nov 2022, at 05:01, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx> wrote:
>
> The series claims "This is beneficial to performance and avoids
> speculation issues.", c/s 8523851dbc4.
>
> That half sentence is literally the sum total of justification given for
> this being related to speculation.
The cover letter, written on 15 Oct 2021, mentions “avoid[ing] indirect
function calls on the hypercall path”. Internal security@ discussions from the
time show that we were talking about Spectre-BHB (AKA BHI) and its impact on
function pointers, specifically those in the hypercall and exception dispatch.
Given that Spectre-BHB wasn’t made public until March 2022, it would have been
a violation of the embargo for Jürgen to go into more detail at that time.
It appears that your view on whether hypercall function call tables are a
vulnerable surface of attack has changed. But given that you once believed
they needed protecting, it’s not unreasonable for other people to think that
they may need protecting; and given that it’s reasonable to think that they may
need protecting, you should at least give a *little bit* of a justification for
why yo believe they don’t, rather than simply falling back to, “There’s no
evidence”.
-George
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