Re: [PATCH stable v4.9 v2] arm64: entry: Place an SB sequence following an ERET instruction

[Florian Fainelli]

On 8/21/20 9:03 AM, Will Deacon wrote:
> On Fri, Aug 07, 2020 at 03:14:29PM +0200, Greg KH wrote:
>> On Thu, Aug 06, 2020 at 01:00:54PM -0700, Florian Fainelli wrote:
>>>
>>>
>>> On 7/20/2020 11:26 AM, Florian Fainelli wrote:
>>>> On 7/20/20 6:04 AM, Greg KH wrote:
>>>>> On Thu, Jul 09, 2020 at 12:50:23PM -0700, Florian Fainelli wrote:
>>>>>> From: Will Deacon <will.deacon@xxxxxxx>
>>>>>>
>>>>>> commit 679db70801da9fda91d26caf13bf5b5ccc74e8e8 upstream
>>>>>>
>>>>>> Some CPUs can speculate past an ERET instruction and potentially perform
>>>>>> speculative accesses to memory before processing the exception return.
>>>>>> Since the register state is often controlled by a lower privilege level
>>>>>> at the point of an ERET, this could potentially be used as part of a
>>>>>> side-channel attack.
>>>>>>
>>>>>> This patch emits an SB sequence after each ERET so that speculation is
>>>>>> held up on exception return.
>>>>>>
>>>>>> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
>>>>>> [florian: Adjust hyp-entry.S to account for the label
>>>>>>  added change to hyp/entry.S]
>>>>>> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
>>>>>> ---
>>>>>> Changes in v2:
>>>>>>
>>>>>> - added missing hunk in hyp/entry.S per Will's feedback
>>>>>
>>>>> What about 4.19.y and 4.14.y trees?  I can't take something for 4.9.y
>>>>> and then have a regression if someone moves to a newer release, right?
>>>>
>>>> Sure, send you candidates for 4.14 and 4.19.
>>>
>>> Greg, did you have a chance to queue those changes for 4.9, 4.14 and 4.19?
>>>
>>> https://lore.kernel.org/linux-arm-kernel/20200720182538.13304-1-f.fainelli@xxxxxxxxx/
>>> https://lore.kernel.org/linux-arm-kernel/20200720182937.14099-1-f.fainelli@xxxxxxxxx/
>>> https://lore.kernel.org/linux-arm-kernel/20200709195034.15185-1-f.fainelli@xxxxxxxxx/
>>
>> Nope, I was waiting for Will's "ack" for these.
> 
> This patch doesn't even build for me (the 'sb' macro is not defined in 4.9),
> and I really wonder why we bother backporting it at all. Nobody's ever shown
> it to be a problem in practice, and it's clear that this is just being
> submitted to tick a box rather than anything else (otherwise it would build,
> right?).

Doh, I completely missed submitting the patch this depended on that's
why I did not notice the build failure locally, sorry about that, what a
shame.

Would not be the same "tick a box" argument be used against your
original submission then? Sure, I have not been able to demonstrate in
real life this was a problem, however the same can be said about a lot
security related fixes.

What if it becomes exploitable in the future, would not it be nice to
have it in a 6 year LTS kernel?

> 
> So I'm not going to Ack any of them. As with a lot of this side-channel
> stuff the cure is far worse than the disease.
Assuming that my v3 does build correctly, which it will, would you be
keen on changing your position?
-- 
Florian