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From: Liran Alon <liran.alon@oracle.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	"Singh, Brijesh" <brijesh.singh@amd.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH 1/2] KVM: SVM: Fix workaround for AMD Errata 1096
Date: Tue, 16 Jul 2019 22:50:11 +0300	[thread overview]
Message-ID: <AAA25F54-FFB6-41F9-98A1-BC07E8B99288@oracle.com> (raw)
In-Reply-To: <20190716194535.GB28096@linux.intel.com>



> On 16 Jul 2019, at 22:45, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> 
> On Tue, Jul 16, 2019 at 09:39:48PM +0200, Paolo Bonzini wrote:
>> On 16/07/19 21:34, Liran Alon wrote:
>>>> When this errata is hit, the CPU will be at CPL3. From hardware
>>>> point-of-view the below sequence happens:
>>>> 
>>>> 1. CPL3 guest hits reserved bit NPT fault (MMIO access)
>>> Why CPU needs to be at CPL3?
>>> The requirement for SMAP should be that this page is user-accessible in guest page-tables.
>>> Think on a case where guest have CR4.SMAP=1 and CR4.SMEP=0.
>>> 
>> 
>> If you are not at CPL3, you'd get a SMAP NPF, not a RSVD NPF.
> 
> I think Liran is right.  When software is executing, the %rip access is
> a code fetch (SMEP), but the ucode assist is a data access (SMAP).
> 
> This likely has only been observed in a CPL3 scenario because no sane OS
> exercises the case of the kernel executing from a user page with SMAP=1
> and SMEP=0.

True. I’m trying to be pedantic and accurate here. :)
I think we should just remove the vCPU CPL check and remain only with the CR4.SMAP check.
Don’t you agree that having a #NPF that returns 0 instruction bytes with DecodeAssist enabled and CR4.SMAP=1
is sufficient for finger-printing this Errata? With which other use-case it’s expected to collide?

-Liran

  reply	other threads:[~2019-07-16 19:50 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-15 20:30 KVM: SVM: Fix workaround for AMD Errata 1096 Liran Alon
2019-07-15 20:30 ` [PATCH 1/2] " Liran Alon
2019-07-16 15:48   ` Singh, Brijesh
2019-07-16 15:56     ` Liran Alon
2019-07-16 16:07       ` Liran Alon
2019-07-16 16:10       ` Singh, Brijesh
2019-07-16 16:20         ` Liran Alon
2019-07-16 16:41           ` Sean Christopherson
2019-07-16 16:56             ` Liran Alon
2019-07-16 17:27               ` Sean Christopherson
2019-07-16 17:27               ` Paolo Bonzini
2019-07-16 17:35                 ` Liran Alon
2019-07-16 19:28                   ` Singh, Brijesh
2019-07-16 19:34                     ` Liran Alon
2019-07-16 19:39                       ` Paolo Bonzini
2019-07-16 19:45                         ` Sean Christopherson
2019-07-16 19:50                           ` Liran Alon [this message]
2019-07-16 19:47                         ` Liran Alon
2019-07-16 19:41                       ` Sean Christopherson
2019-07-16 19:52                         ` Liran Alon
2019-07-16 20:02                       ` Singh, Brijesh
2019-07-16 20:07                         ` Sean Christopherson
2019-07-16 20:13                           ` Paolo Bonzini
2019-07-16 20:09                         ` Liran Alon
2019-07-16 20:27                           ` Singh, Brijesh
2019-07-16 20:54                             ` Sean Christopherson
2019-07-16 21:53                               ` Liran Alon
2019-07-16 18:05           ` Singh, Brijesh
2019-07-16 18:06             ` Singh, Brijesh
2019-07-15 20:30 ` [PATCH 2/2] KVM: x86: Rename need_emulation_on_page_fault() to handle_no_insn_on_page_fault() Liran Alon
2019-07-16 15:48   ` Sean Christopherson
2019-07-16 16:01     ` Liran Alon
2019-07-16 16:10       ` Sean Christopherson
2019-07-16 19:33         ` Singh, Brijesh

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