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



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



 


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