From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack
Date: Thu, 25 Jun 2020 09:25:40 -0700 [thread overview]
Message-ID: <20200625162540.GC3437@linux.intel.com> (raw)
In-Reply-To: <1cebc562-89e9-3806-bb3c-771946fc64f3@redhat.com>
On Thu, Jun 25, 2020 at 10:09:13AM +0200, Paolo Bonzini wrote:
> On 25/06/20 08:15, Sean Christopherson wrote:
> > IMO, kvm_cpuid() is simply buggy. If KVM attempts to access a non-existent
> > MSR then it darn well should warn.
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 8a294f9747aa..7ef7283011d6 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -1013,7 +1013,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> > *ebx = entry->ebx;
> > *ecx = entry->ecx;
> > *edx = entry->edx;
> > - if (function == 7 && index == 0) {
> > + if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) &&
> > + (vcpu->arch.arch_capabilities & ARCH_CAP_TSX_CTRL_MSR)) {
> > u64 data;
> > if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
> > (data & TSX_CTRL_CPUID_CLEAR))
> >
>
> That works too, but I disagree that warning is the correct behavior
> here. It certainly should warn as long as kvm_get_msr blindly returns
> zero. However, for a guest it's fine to access a potentially
> non-existent MSR if you're ready to trap the #GP, and the point of this
> series is to let cpuid.c or any other KVM code do the same.
I get the "what" of the change, and even the "why" to some extent, but I
dislike the idea of supporting/encouraging blind reads/writes to MSRs.
Blind writes are just asking for problems, and suppressing warnings on reads
is almost guaranteed to be suppressing a KVM bug.
Case in point, looking at the TSX thing again, I actually think the fix
should be:
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 5eb618dbf211..64322446e590 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1013,9 +1013,9 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
*ebx = entry->ebx;
*ecx = entry->ecx;
*edx = entry->edx;
- if (function == 7 && index == 0) {
+ if (function == 7 && index == 0 && (*ebx | (F(RTM) | F(HLE))) {
u64 data;
- if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
+ if (!kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data) &&
(data & TSX_CTRL_CPUID_CLEAR))
*ebx &= ~(F(RTM) | F(HLE));
}
On VMX, MSR_IA32_TSX_CTRL will be added to the so called shared MSR array
regardless of whether or not it is being advertised to userspace (this is
a bug in its own right). Using the host_initiated variant means KVM will
incorrectly bypass VMX's ARCH_CAP_TSX_CTRL_MSR check, i.e. incorrectly
clear the bits if userspace is being weird and stuffed MSR_IA32_TSX_CTRL
without advertising it to the guest.
In short, the whole MSR_IA32_TSX_CTRL implementation seems messy and this
is just papering over that mess. The correct fix is to invoke setup_msrs()
on writes to MSR_IA32_ARCH_CAPABILITIES, filtering MSR_IA32_TSX_CTRL out of
shared MSRs when it's not advertised, and change kvm_cpuid() to use the
unpriveleged variant.
TSC_CTRL aside, if we insist on pointing a gun at our foot at some point,
this should be a dedicated flavor of MSR access, e.g. msr_data.kvm_initiated,
so that it at least requires intentionally loading the gun.
next prev parent reply other threads:[~2020-06-25 16:25 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-22 22:04 [PATCH 0/2] KVM: X86: A few fixes around ignore_msrs Peter Xu
2020-06-22 22:04 ` [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack Peter Xu
2020-06-25 6:15 ` Sean Christopherson
2020-06-25 8:09 ` Paolo Bonzini
2020-06-25 16:25 ` Sean Christopherson [this message]
2020-06-25 17:45 ` Sean Christopherson
2020-06-25 18:44 ` Paolo Bonzini
2020-06-26 15:56 ` Sean Christopherson
2020-06-26 17:37 ` Peter Xu
2020-06-26 17:46 ` Sean Christopherson
2020-06-26 18:07 ` Peter Xu
2020-06-26 18:18 ` Sean Christopherson
2020-06-26 19:11 ` Peter Xu
2020-06-27 14:24 ` Paolo Bonzini
2020-06-30 15:47 ` Sean Christopherson
2020-07-09 18:22 ` Peter Xu
2020-07-09 18:24 ` Paolo Bonzini
2020-07-09 18:34 ` Peter Xu
2020-07-09 19:24 ` Sean Christopherson
2020-07-09 21:09 ` Peter Xu
2020-07-09 21:26 ` Sean Christopherson
2020-07-09 21:50 ` Peter Xu
2020-07-09 22:11 ` Paolo Bonzini
2020-07-10 4:58 ` Sean Christopherson
2020-06-22 22:04 ` [PATCH 2/2] KVM: X86: Do the same ignore_msrs check for feature msrs Peter Xu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200625162540.GC3437@linux.intel.com \
--to=sean.j.christopherson@intel.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=pbonzini@redhat.com \
--cc=peterx@redhat.com \
--cc=vkuznets@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).