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Re: [Xen-devel] [PATCH v3 2/7] xen/nospec: Use always_inline to fix code gen for evaluate_nospec


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 29 Oct 2019 16:53:21 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 29 Oct 2019 16:53:35 +0000
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On 25/10/2019 13:03, Jan Beulich wrote:
> On 23.10.2019 15:58, Andrew Cooper wrote:
>> evaluate_nospec() is incredibly fragile, and this is one giant bodge.
>>
>> To correctly protect jumps, the generated code needs to be of the form:
>>
>>     cmp/test <cond>
>>     jcc 1f
>>     lfence
>>     ...
>>  1: lfence
>>     ...
>>
>> Critically, the lfence must be at the head of both basic blocks, later in the
>> instruction stream than the conditional jump in need of protection.
>>
>> When a static inline is involved, the optimiser decides to be clever and
>> rearranges the code as:
>>
>>  pred:
>>     lfence
>>     <calculate cond>
>>     ret
>>
>>     call pred
>>     cmp $0, %eax
>>     jcc 1f
>>     ...
>>  1: ...
>>
>> which breaks the speculative safety.
> Aiui "pred" is a non-inlined static inline here. There's no "optimiser decides
> to be clever" in this case imo - it all is a result of not inlining, when the
> construct in evaluate_nospec() is specifically assuming this wouldn't happen.
> Therefore I continue to think that the description is misleading.
>
>> Any use of evaluate_nospec() needs all static inline predicates which use it
>> to be declared always_inline to prevent the optimiser having the flexibility
>> to generate unsafe code.
> I agree with this part.

How about:

When the compiler chooses to out-of-line the condition calculation (e.g. by
not inlining a predicate), the code layout can end up as:
   
 pred:
    lfence
    <calculate cond>
    ret
   
    call pred
    cmp $0, %eax
    jcc 1f
    ...
 1: ...
   
which breaks the speculative safety, as the lfences are earlier in the
instruction stream than the jump in need of protection.

?

~Andrew

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