Re: [PATCH v1 5/9] KVM: x86: Add new hypercall to lock control registers

From: Kees Cook <keescook@chromium.org>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: "Wei Liu" <wei.liu@kernel.org>, "Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Sean Christopherson" <seanjc@google.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Alexander Graf" <graf@amazon.com>,
	"Forrest Yuan Yu" <yuanyu@google.com>,
	"James Morris" <jamorris@linux.microsoft.com>,
	"John Andersen" <john.s.andersen@intel.com>,
	"Madhavan T . Venkataraman" <madvenka@linux.microsoft.com>,
	"Marian Rotariu" <marian.c.rotariu@gmail.com>,
	"Mihai Donțu" <mdontu@bitdefender.com>,
	"Nicușor Cîțu" <nicu.citu@icloud.com>,
	"Rick Edgecombe" <rick.p.edgecombe@intel.com>,
	"Thara Gopinath" <tgopinath@microsoft.com>,
	"Will Deacon" <will@kernel.org>,
	"Zahra Tarkhani" <ztarkhani@microsoft.com>,
	"Ștefan Șicleru" <ssicleru@bitdefender.com>,
	dev@lists.cloudhypervisor.org, kvm@vger.kernel.org,
	linux-hardening@vger.kernel.org, linux-hyperv@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org, qemu-devel@nongnu.org,
	virtualization@lists.linux-foundation.org, x86@kernel.org,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v1 5/9] KVM: x86: Add new hypercall to lock control registers
Date: Tue, 30 May 2023 16:16:09 -0700	[thread overview]
Message-ID: <202305301614.BF8D80D3D5@keescook> (raw)
In-Reply-To: <901ff104-215c-8e81-fbae-5ecd8fa94449@digikod.net>

On Mon, May 29, 2023 at 06:48:03PM +0200, Mickaël Salaün wrote:
> 
> On 08/05/2023 23:11, Wei Liu wrote:
> > On Fri, May 05, 2023 at 05:20:42PM +0200, Mickaël Salaün wrote:
> > > This enables guests to lock their CR0 and CR4 registers with a subset of
> > > X86_CR0_WP, X86_CR4_SMEP, X86_CR4_SMAP, X86_CR4_UMIP, X86_CR4_FSGSBASE
> > > and X86_CR4_CET flags.
> > > 
> > > The new KVM_HC_LOCK_CR_UPDATE hypercall takes two arguments.  The first
> > > is to identify the control register, and the second is a bit mask to
> > > pin (i.e. mark as read-only).
> > > 
> > > These register flags should already be pinned by Linux guests, but once
> > > compromised, this self-protection mechanism could be disabled, which is
> > > not the case with this dedicated hypercall.
> > > 
> > > Cc: Borislav Petkov <bp@alien8.de>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: H. Peter Anvin <hpa@zytor.com>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > Cc: Sean Christopherson <seanjc@google.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> > > Cc: Wanpeng Li <wanpengli@tencent.com>
> > > Signed-off-by: Mickaël Salaün <mic@digikod.net>
> > > Link: https://lore.kernel.org/r/20230505152046.6575-6-mic@digikod.net
> > [...]
> > >   	hw_cr4 = (cr4_read_shadow() & X86_CR4_MCE) | (cr4 & ~X86_CR4_MCE);
> > >   	if (is_unrestricted_guest(vcpu))
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index ffab64d08de3..a529455359ac 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -7927,11 +7927,77 @@ static unsigned long emulator_get_cr(struct x86_emulate_ctxt *ctxt, int cr)
> > >   	return value;
> > >   }
> > > +#ifdef CONFIG_HEKI
> > > +
> > > +extern unsigned long cr4_pinned_mask;
> > > +
> > 
> > Can this be moved to a header file?
> 
> Yep, but I'm not sure which one. Any preference Kees?

Uh, er, I was never expecting that mask to be non-static. ;) To that
end, how about putting it in arch/x86/kvm/x86.h ?

-- 
Kees Cook