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

[Maciej S. Szmigiero]

On 21.03.2018 15:05, Dave Hansen wrote:
> On 03/20/2018 04:17 AM, Maciej S. Szmigiero wrote:
>> If we run on a CPU that does not have IBPB support RSB entries from one
>> userspace process can influence 'ret' target prediction in another
>> userspace process after a context switch.
>>
>> 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,
> 
> The assumption thus far (good or bad) is that everything will get a
> microcode update.  I actually don't know for sure if RSB manipulation is
> effective on old microcode before Skylake.  I'm pretty sure it has not
> been documented publicly.
> 
> How did you decide that this is an effective mitigation?
> 
A RSB overwrite is already being done even on pre-Skylake Intel CPUs on
VMEXIT to protect the host from the guest, regardless of the microcode
version.

But I see that an Intel guidance document published last month about
retpolines says that "RET has this [predictable speculative] behavior on
all processors (...) microarchitecture codename Broadwell and earlier
when updated with the latest microcode".
This suggests that updated microcode may be needed for protection anyway
on such CPUs - as you say.  Such update (hopefully) brings IBPB
support, too, so I agree that the change introduced by this patch can be
skipped on Intel CPUs.

Maciej