Re: [PATCH] x86/speculation: Fill the RSB on context switch also on non-IBPB CPUs

["Maciej S. Szmigiero" <mail@maciej.szmigiero.name>]

On 22.03.2018 00:30, Dave Hansen wrote:
> On 03/20/2018 04:17 AM, Maciej S. Szmigiero wrote:
>> Since it is unlikely that existing RSB entries from the previous task match
>> the new task call stack we can use the existing unconditional
>> RSB-filling-on-context-switch infrastructure to protect against such
>> userspace-to-userspace attacks.
>>
>> This patch brings a change in behavior only for the following CPU types:
>> * Intel pre-Skylake CPUs without updated microcode,
>> * AMD Family 15h model >60h, Family 17h CPUs without updated microcode.
>>
>> Other CPU types either already do the RSB filling on context switch for
>> other reasons or do support IBPB for more complete userspace-to-userspace
>> protection.
> 
> I think I misunderstood your reasoning a bit.  Let me see if I can
> restate the problem a bit.
> 
> IBPB provides provides userspace-to-userspace protection because it
> prevents all indirect branch predictions after the barrier from being
> controlled by software executed before the barrier.  We only use IBPB
> for KVM and when processes clear their dumpable flag.
> 
> You're saying that, even if we don't have IBPB, we can do *some*
> userspace-to-userspace protection with RSB manipulation.  The RSB
> manipulation obviously only helps 'RET' instructions and not JMP/CALL,
> but it does do *something* useful.
> 
> Is that right?

Yes.

As far as I understand the issue this should provide a good protection
for userspace processes that were recompiled with retpolines as they
won't have any indirect jumps and calls.

> Do you really want this behavior on all context switches?  We don't do
> IBPB on all context switches, only the ones where we are switching *to*
> a non-dumpable process.
> 
> Do you perhaps want to do RSB manipulation in lieu of IBPB when
> switching *to* a non-dumpable process and IBPB is not available?
> 

Is it worth differentiating such processes in this case?
IBPB is supposed to be very expensive so certainly it is worthwhile
to do it only for high-value processes (=non-dumpable).

However, it is unlikely that existing RSB entries from the previous
task match the new task call stack anyway.
We already do unconditional RSB-filling-on-context-switch in many
cases.

Maciej