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

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Message-ID: <3ac6da4a8586014925057a413ce46407b9699fa9.camel@amd.com>
Date: Mon, 4 Nov 2024 17:22:12 +0000
From: "Shah, Amit" <Amit.Shah@....com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "dave.hansen@...el.com"
	<dave.hansen@...el.com>, "linux-kernel@...r.kernel.org"
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	<David.Kaplan@....com>
Subject: Re: [PATCH 1/2] x86: cpu/bugs: add support for AMD ERAPS feature

On Mon, 2024-11-04 at 08:26 -0800, Dave Hansen wrote:
> On 11/4/24 08:13, Shah, Amit wrote:
> > I want to justify that not setting X86_FEATURE_RSB_CTXSW is still
> > doing
> > the right thing, albeit in hardware.
> 
> Let's back up a bit.
> 
> In the kernel, we have security concerns if RSB contents remain
> across
> context switches.  If process A's RSB entries are left and then
> process
> B uses them, there's a problem.
> 
> Today, we mitigate that issue with manual kernel RSB state zapping on
> context switches (X86_FEATURE_RSB_CTXSW).
> 
> You're saying that this fancy new ERAPS feature includes a new
> mechanism
> to zap RSB state.  But that only triggers "each time a TLB flush
> happens".
> 
> So what you're saying above is that you are concerned about RSB
> contents
> sticking around across context switches.  But instead of using
> X86_FEATURE_RSB_CTXSW, you believe that the new TLB-flush-triggered
> ERAPS flush can be used instead.
> 
> Are we all on the same page so far?

All good so far.

> 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?  Does
this now cover all the cases?

		Amit