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Message-ID: <Zy0__5YB9F5d0eZn@google.com>
Date: Thu, 7 Nov 2024 14:32:31 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Zack Rusin <zack.rusin@...adcom.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
Doug Covelli <doug.covelli@...adcom.com>, Jonathan Corbet <corbet@....net>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
Arnaldo Carvalho de Melo <acme@...hat.com>, Isaku Yamahata <isaku.yamahata@...el.com>,
Joel Stanley <joel@....id.au>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls
On Mon, Nov 04, 2024, Zack Rusin wrote:
> On Mon, Nov 4, 2024 at 5:13 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On Wed, Oct 30, 2024 at 4:35 AM Zack Rusin <zack.rusin@...adcom.com> wrote:
> > >
> > > VMware products handle hypercalls in userspace. Give KVM the ability
> > > to run VMware guests unmodified by fowarding all hypercalls to the
> > > userspace.
> > >
> > > Enabling of the KVM_CAP_X86_VMWARE_HYPERCALL_ENABLE capability turns
> > > the feature on - it's off by default. This allows vmx's built on top
> > > of KVM to support VMware specific hypercalls.
> >
> > Hi Zack,
>
> Hi, Paolo.
>
> Thank you for looking at this.
>
> > is there a spec of the hypercalls that are supported by userspace? I
> > would like to understand if there's anything that's best handled in
> > the kernel.
>
> There's no spec but we have open headers listing the hypercalls.
> There's about a 100 of them (a few were deprecated), the full
> list starts here:
> https://github.com/vmware/open-vm-tools/blob/739c5a2f4bfd4cdda491e6a6f6869d88c0bd6972/open-vm-tools/lib/include/backdoor_def.h#L97
> They're not well documented, but the names are pretty self-explenatory.
At a quick glance, this one needs to be handled in KVM:
BDOOR_CMD_VCPU_MMIO_HONORS_PAT
and these probably should be in KVM:
BDOOR_CMD_GETTIME
BDOOR_CMD_SIDT
BDOOR_CMD_SGDT
BDOOR_CMD_SLDT_STR
BDOOR_CMD_GETTIMEFULL
BDOOR_CMD_VCPU_LEGACY_X2APIC_OK
BDOOR_CMD_STEALCLOCK
and these maybe? (it's not clear what they do, from the name alone)
BDOOR_CMD_GET_VCPU_INFO
BDOOR_CMD_VCPU_RESERVED
> > If we allow forwarding _all_ hypercalls to userspace, then people will
> > use it for things other than VMware and there goes all hope of
> > accelerating stuff in the kernel in the future.
To some extent, that ship has sailed, no? E.g. do KVM_XEN_HVM_CONFIG with
KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL set, and userspace can intercept pretty much
all hypercalls with very few side effects.
> > So even having _some_ checks in the kernel before going out to
> > userspace would keep that door open, or at least try.
>
> Doug just looked at this and I think I might have an idea on how to
> limit the scope at least a bit: if you think it would help we could
> limit forwarding of hypercalls to userspace only to those that that
> come with a BDOOR_MAGIC (which is 0x564D5868) in eax. Would that help?
I don't think it addresses Paolo's concern (if I understood Paolo's concern
correctly), but it would help from the perspective of allowing KVM to support
VMware hypercalls and Xen/Hyper-V/KVM hypercalls in the same VM.
I also think we should add CONFIG_KVM_VMWARE from the get-go, and if we're feeling
lucky, maybe even retroactively bury KVM_CAP_X86_VMWARE_BACKDOOR behind that
Kconfig. That would allow limiting the exposure to VMware specific code, e.g. if
KVM does end up handling hypercalls in-kernel. And it might deter abuse to some
extent.
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