linux-kernel - Re: [PATCH v8 00/26] Enable CET Virtualization

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Date: Fri, 5 Jan 2024 17:52:20 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yang, Weijiang" <weijiang.yang@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
	"peterz@...radead.org" <peterz@...radead.org>, "john.allen@....com"
	<john.allen@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>
Subject: Re: [PATCH v8 00/26] Enable CET Virtualization

On Fri, 2024-01-05 at 08:21 -0800, Sean Christopherson wrote:
> No, do not inject #UD or do anything else that deviates from
> architecturally
> defined behavior.

Here is a, at least partial, list of CET touch points I just created by
searching the SDM:
1. The emulator SW fetch with TRACKER=1
2. CALL, RET, JMP, IRET, INT, SYSCALL, SYSENTER, SYSEXIT, SYSRET
3. Task switching
4. The new CET instructions (which I guess should be handled by
default): CLRSSBSY, INCSSPD, RSTORSSP, SAVEPREVSSP, SETSSBSYY, WRSS,
WRUSS

Not all of those are security checks, but would have some functional
implications. It's still not clear to me if this could happen naturally
(the TDP shadowing stuff), or only via strange attacker behavior. If we
only care about the attacker case, then we could have a smaller list.

It also sounds like the instructions in 2 could maybe be filtered by
mode instead of caring about CET being enabled. But maybe it's not good
to mix the CET problem with the bigger emulator issues. Don't know.