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