Re: [Xen-devel] [PATCH 2/2] x86/hvm: Don't intercept #UD exceptions in general

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/01/16 18:49, Boris Ostrovsky wrote:
> On 01/27/2016 01:11 PM, Andrew Cooper wrote:
>> c/s 0f1cb96e "x86 hvm: Allow cross-vendor migration" caused HVM
>> domains to
>> unconditionally intercept #UD exceptions.  While cross-vendor
>> migration is
>> cool as a demo, it is extremely niche.
>>
>> Intercepting #UD allows userspace code in a multi-vcpu guest to execute
>> arbitrary instructions in the x86 emulator by having one thread
>> execute a ud2a
>> instruction, and having a second thread rewrite the instruction
>> before the
>> emulator performs an instruction fetch.
>>
>> XSAs 105, 106 and 110 are all examples where guest userspace can use
>> bugs in
>> the x86 emulator to compromise security of the domain, either by
>> privilege
>> escalation or causing a crash.
>>
>> c/s 2d67a7a4 "x86: synchronize PCI config space access decoding"
>> introduced (amongst other things) a per-domain vendor, based on the
>> guests
>> cpuid policy.
>>
>> Use the per-guest vendor to enable #UD interception only when a
>> domain is
>> configured for a vendor different to the current hardware.  (#UD
>> interception
>> is also enabled if hvm_fep is specified on the Xen command line. 
>> This is a
>> debug-only option whose entire purpose is for testing the x86 emulator.)
>>
>> As a result, the overwhelming majority of usecases now have #UD
>> interception
>> disabled, removing an attack surface for malicious guest userspace.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
>> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>> CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
>> CC: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@xxxxxxx>
>> ---
>>   xen/arch/x86/domctl.c         | 18 ++++++++++++++++++
>>   xen/arch/x86/hvm/hvm.c        |  6 ++----
>>   xen/arch/x86/hvm/svm/svm.c    | 13 +++++++++++++
>>   xen/arch/x86/hvm/svm/vmcb.c   |  1 +
>>   xen/arch/x86/hvm/vmx/vmcs.c   |  1 +
>>   xen/arch/x86/hvm/vmx/vmx.c    | 15 +++++++++++++++
>>   xen/include/asm-x86/hvm/hvm.h | 15 ++++++++++++++-
>>   7 files changed, 64 insertions(+), 5 deletions(-)
>>
>> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
>> index 1d71216..1084e82 100644
>> --- a/xen/arch/x86/domctl.c
>> +++ b/xen/arch/x86/domctl.c
>> @@ -65,8 +65,20 @@ static void update_domain_cpuid_info(struct domain
>> *d,
>>                   .ecx = ctl->ecx
>>               }
>>           };
>> +        int old_vendor = d->arch.x86_vendor;
>>             d->arch.x86_vendor = get_cpu_vendor(vendor_id.str,
>> gcv_guest);
>> +
>> +        if ( is_hvm_domain(d) && (d->arch.x86_vendor != old_vendor) )
>> +        {
>> +            struct vcpu *v;
>> +
>> +            domain_pause(d);
>> +            for_each_vcpu( d, v )
>> +                hvm_update_guest_vendor(v);
>> +            domain_unpause(d);
>> +        }
>> +
>>           break;
>>       }
>
> Not specific to this patch, but shouldn't we pause/unpause domain for
> the whole routine?

Not specifically, although that might be better lonterm.

In practice, this hypercall is only made as part of domain construction,
and never at domain runtime.

>
>
>>   @@ -707,6 +719,12 @@ long arch_do_domctl(
>>           xen_domctl_cpuid_t *ctl = &domctl->u.cpuid;
>>           cpuid_input_t *cpuid, *unused = NULL;
>>   +        if ( d == currd ) /* no domain_pause() */
>> +        {
>> +            ret = -EINVAL;
>> +            break;
>> +        }
>> +
>>           for ( i = 0; i < MAX_CPUID_INPUT; i++ )
>>           {
>>               cpuid = &d->arch.cpuids[i];
>
> ...
>
>>     /* Xen command-line option to enable altp2m */
>> diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
>> index 953e0b5..44a1250 100644
>> --- a/xen/arch/x86/hvm/svm/svm.c
>> +++ b/xen/arch/x86/hvm/svm/svm.c
>> @@ -597,6 +597,18 @@ static void svm_update_guest_efer(struct vcpu *v)
>>       vmcb_set_efer(vmcb, new_efer);
>>   }
>>   +static void svm_update_guest_vendor(struct vcpu *v)
>> +{
>> +    struct arch_svm_struct *arch_svm = &v->arch.hvm_svm;
>> +    struct vmcb_struct *vmcb = arch_svm->vmcb;
>> +
>> +    if ( opt_hvm_fep ||
>> +         (v->domain->arch.x86_vendor != boot_cpu_data.x86_vendor) )
>> +        vmcb->_exception_intercepts |= (1U << TRAP_invalid_op);
>> +    else
>> +        vmcb->_exception_intercepts &= ~(1U << TRAP_invalid_op);
>> +}
>
> I think you need to clear clean bits here (at least bit 0).

Hmm - looks like I copied some other code in need of fixing.  I will see
what I can do.

~Andrew

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