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

[Julien Grall <julien@xxxxxxx>]

Hi,

On 16/06/2020 22:24, Stefano Stabellini wrote:
> On Tue, 16 Jun 2020, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>
> 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