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

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Date:   Mon, 29 May 2023 18:48:03 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Alexander Graf <graf@...zon.com>,
        Forrest Yuan Yu <yuanyu@...gle.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        John Andersen <john.s.andersen@...el.com>,
        "Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>,
        Marian Rotariu <marian.c.rotariu@...il.com>,
        Mihai Donțu <mdontu@...defender.com>,
        Nicușor Cîțu <nicu.citu@...oud.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Thara Gopinath <tgopinath@...rosoft.com>,
        Will Deacon <will@...nel.org>,
        Zahra Tarkhani <ztarkhani@...rosoft.com>,
        Ștefan Șicleru <ssicleru@...defender.com>,
        dev@...ts.cloudhypervisor.org, kvm@...r.kernel.org,
        linux-hardening@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v1 5/9] KVM: x86: Add new hypercall to lock control
 registers


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@...en8.de>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: H. Peter Anvin <hpa@...or.com>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Madhavan T. Venkataraman <madvenka@...ux.microsoft.com>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Sean Christopherson <seanjc@...gle.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
>> Cc: Wanpeng Li <wanpengli@...cent.com>
>> Signed-off-by: Mickaël Salaün <mic@...ikod.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?


> 
>> +static int heki_lock_cr(struct kvm *const kvm, const unsigned long cr,
>> +			unsigned long pin)
>> +{
>> +	if (!pin)
>> +		return -KVM_EINVAL;
>> +
>> +	switch (cr) {
>> +	case 0:
>> +		/* Cf. arch/x86/kernel/cpu/common.c */
>> +		if (!(pin & X86_CR0_WP))
>> +			return -KVM_EINVAL;
>> +
>> +		if ((read_cr0() & pin) != pin)
>> +			return -KVM_EINVAL;
>> +
>> +		atomic_long_or(pin, &kvm->heki_pinned_cr0);
>> +		return 0;
>> +	case 4:
>> +		/* Checks for irrelevant bits. */
>> +		if ((pin & cr4_pinned_mask) != pin)
>> +			return -KVM_EINVAL;
>> +
> 
> It is enforcing the host mask on the guest, right? If the guest's set is a
> super set of the host's then it will get rejected.
> 
> 
>> +		/* Ignores bits not present in host. */
>> +		pin &= __read_cr4();
>> +		atomic_long_or(pin, &kvm->heki_pinned_cr4);

We assume that the host's mask is a superset of the guest's mask. I 
guess we should check the absolute supported bits instead, even if it 
would be weird for the host to not support these bits.


>> +		return 0;
>> +	}
>> +	return -KVM_EINVAL;
>> +}
>> +
>> +int heki_check_cr(const struct kvm *const kvm, const unsigned long cr,
>> +		  const unsigned long val)
>> +{
>> +	unsigned long pinned;
>> +
>> +	switch (cr) {
>> +	case 0:
>> +		pinned = atomic_long_read(&kvm->heki_pinned_cr0);
>> +		if ((val & pinned) != pinned) {
>> +			pr_warn_ratelimited(
>> +				"heki-kvm: Blocked CR0 update: 0x%lx\n", val);
> 
> I think if the message contains the VM and VCPU identifier it will
> become more useful.

Indeed, and this should be the case for all log messages, but I'd left 
that for future work. ;) I'll update the logs for the next series with a 
new kvm_warn_ratelimited() helper using VCPU's PID.