Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

Shah, Amit
2024-11-05T10:39:45Z

On Mon, 2024-11-04 at 09:45 -0800, Dave Hansen wrote:
> On 11/4/24 09:22, Shah, Amit wrote:
> > > I think you're wrong.  We can't depend on ERAPS for this.  Linux
> > > doesn't flush the TLB on context switches when PCIDs are in play.
> > > Thus, ERAPS won't flush the RSB and will leave bad state in there
> > > and will leave the system vulnerable.
> > >
> > > Or what am I missing?
> > I just received confirmation from our hardware engineers on this
> > too:
> >
> > 1. the RSB is flushed when CR3 is updated
> > 2. the RSB is flushed when INVPCID is issued (except type 0 -
> > single
> > address).
> >
> > I didn't mention 1. so far, which led to your question, right?
>
> Not only did you not mention it, you said something _completely_
> different.  So, where the documentation for this thing?  I dug
> through
> the 57230 .zip file and I see the CPUID bit:
>
> 24 ERAPS. Read-only. Reset: 1. Indicates support for
> enhanced
>   return address predictor security.
>
> but nothing telling us how it works.

I'm expecting the APM update come out soon, but I have put together

https://amitshah.net/2024/11/eraps-reduces-software-tax-for-hardware-bugs/

based on information I have. I think it's mostly consistent with what
I've said so far - with the exception of the mov-CR3 flush only
confirmed yesterday.

> > Does this now cover all the cases?
>
> Nope, it's worse than I thought.  Look at:
>
> > SYM_FUNC_START(__switch_to_asm)
> ...
> >         FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS,
> > X86_FEATURE_RSB_CTXSW
>
> which does the RSB fill at the same time it switches RSP.
>
> So we feel the need to flush the RSB on *ALL* task switches.  That
> includes switches between threads in a process *AND* switches over to
> kernel threads from user ones.

(since these cases are the same as those listed below, I'll only reply
in one place)

> So, I'll flip this back around.  Today, X86_FEATURE_RSB_CTXSW zaps
> the
> RSB whenever RSP is updated to a new task stack.  Please convince me
> that ERAPS provides superior coverage or is unnecessary in all the
> possible combinations switching between:
>
> different thread, same mm

This case is the same userspace process with valid addresses in the RSB
for that process. An invalid speculation isn't security sensitive,
just a misprediction that won't be retired. So we are good here.

> user=>kernel, same mm
> kernel=>user, same mm

user-kernel is protected with SMEP. Also, we don't call
FILL_RETURN_BUFFER for these switches?

> different mm (we already covered this)
>
> Because several of those switches can happen without a CR3 write or
> INVPCID.

(that covers all of them IIRC)

Amit