linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20211227183253.45a03ca2@redhat.com>
Date:   Mon, 27 Dec 2021 18:32:53 +0100
From:   Igor Mammedov <imammedo@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

On Fri, 26 Nov 2021 13:20:28 +0100
Paolo Bonzini <pbonzini@...hat.com> wrote:

> On 11/22/21 18:58, Vitaly Kuznetsov wrote:
> > -	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> > -	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> > -	 * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> > -	 * faults due to reusing SPs/SPTEs.  Alert userspace, but otherwise
> > -	 * sweep the problem under the rug.
> > -	 *
> > -	 * KVM's horrific CPUID ABI makes the problem all but impossible to
> > -	 * solve, as correctly handling multiple vCPU models (with respect to
> > -	 * paging and physical address properties) in a single VM would require
> > -	 * tracking all relevant CPUID information in kvm_mmu_page_role.  That
> > -	 * is very undesirable as it would double the memory requirements for
> > -	 * gfn_track (see struct kvm_mmu_page_role comments), and in practice
> > -	 * no sane VMM mucks with the core vCPU model on the fly.
> > +	 * Changing guest CPUID after KVM_RUN is forbidden, see the comment in
> > +	 * kvm_arch_vcpu_ioctl().
> >   	 */  
> 
> The second part of the comment still applies to kvm_mmu_after_set_cpuid 
> more than to kvm_arch_vcpu_ioctl().
> 
> >  		r = -EFAULT;
> > [...]
> > +		if (vcpu->arch.last_vmentry_cpu != -1)
> > +			goto out;
> > +
> >  		if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
> >  			goto out;
> >  		r = kvm_vcpu_ioctl_set_cpuid(vcpu, &cpuid, cpuid_arg->entries);  
> 
> This should be an EINVAL.
> 
> Tweaked and queued nevertheless, thanks.

it seems this patch breaks VCPU hotplug, in scenario:

  1. hotunplug existing VCPU (QEMU stores VCPU file descriptor in parked cpus list)
  2. hotplug it again (unsuspecting QEMU reuses stored file descriptor when recreating VCPU)

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=2028337#c11

> 
> Paolo
>