Re: [PATCH] x86/bugs: protect against userspace-userspace spectreRSB

[Tim Chen]

On 07/25/2018 10:11 AM, Josh Poimboeuf wrote:
> On Wed, Jul 25, 2018 at 03:50:44PM +0200, Jiri Kosina wrote:
>> On Wed, 25 Jul 2018, Josh Poimboeuf wrote:
>>
>>>> The article "Spectre Returns! Speculation Attacks using the Return Stack
>>>> Buffer" [1] describes two new (sub-)variants of spectrev2-like attack,
>>>> making use solely of the RSB contents even on CPUs that don't fallback to
>>>> BTB on RSB underflow (Skylake+).
>>>>
>>>> Mitigate userspace-userspace attacks by always unconditionally filling RSB on
>>>> context switch when generic spectrev2 mitigation has been enabled.
>>>>
>>>> [1] https://arxiv.org/pdf/1807.07940.pdf
>>>>
>>>> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
>>>
>>> While I generally agree with this patch, isn't it odd that we would do
>>> RSB filling on every context switch, but almost never do IBPB?
>>
>> Yeah, I have actually been wondering exactly the same, but that's what we 
>> have been doing so far on SKL+, so I didn't really want to mix this aspect 
>> in.
>>
>> I actually believe that in the name of consistency we should've been doing 
>> the RSB fills under the same conditions we're issuing IBPB even on SKL+; I 
>> can resend a patch that re-adjusts that, if that's the consensus.
> 
> True, in theory it might make more sense to only fill RSB when doing an
> IBPB.  But given the current state of almost never doing IBPB, that
> would be pointless.  RSB is cheap enough that we should just do it
> unconditionally on context switch.
> 
> BTW, I've heard that IBPB actually flushes RSB, though I haven't seen
> that officially documented anywhere.  Not that it matters given the
> current IBPB code.

I think that's correct.  IBPB does flushes the RSB.  But doing
RSB stuffing will be cheaper.

Tim

> 
> BTW^2, there was some discussion a few months back about offloading the
> "when to IBPB" decision to security modules, though I don't think I've
> ever seen official patches for that.
> 
> All that said, this patch is fine until if/when the IBPB strategy gets
> figured out.  RSB filling is cheap.
> 
> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>