Re: [PATCH 1/2] xen/arm: entry: Place a speculation barrier following an ret instruction

[Julien Grall <julien@xen.org>]

Hi,

On 16/06/2020 22:24, Stefano Stabellini wrote:
> On Tue, 16 Jun 2020, Julien Grall wrote:
>> From: Julien Grall <jgrall@amazon.com>
>>
>> Some CPUs can speculate past a RET instruction and potentially perform
>> speculative accesses to memory before processing the return.
>>
>> There is no known gadget available after the RET instruction today.
>> However some of the registers (such as in check_pending_guest_serror())
>> may contain a value provided the guest.
>                                ^ by
> 
> 
>> In order to harden the code, it would be better to add a speculation
>> barrier after each RET instruction. The performance is meant to be
>> negligeable as the speculation barrier is not meant to be archicturally
>> executed.
>>
>> Note that on arm32, the ldmia instruction will act as a return from the
>> function __context_switch(). While the whitepaper doesn't suggest it is
>> possible to speculate after the instruction, add preventively a
>> speculation barrier after it as well.
>>
>> This is part of the work to mitigate straight-line speculation.
>>
>> Signed-off-by: Julien Grall <jgrall@amazon.com>
> 
> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
> 
> I did a compile-test on the patch too.
> 
> 
>> ---
>>
>> I am still unsure whether we preventively should add a speculation barrier
>> preventively after all the RET instructions in arm*/lib/. The smc call be
>> taken care in a follow-up patch.
> 
> SMC is great to have but it seems to be overkill to do the ones under
> lib/.
 From my understanding, the compiler will add a speculation barrier 
preventively after each 'ret' when the mitigation are turned on.So it 
feels to me we want to follow the same approach.

Obviously, we can avoid them but I would like to have a justification 
for not adding them (nothing is overkilled against speculation ;)).

Cheers,

-- 
Julien Grall