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[Xen-devel] [PATCH for-4.5 v4] x86/hvm: Further restrict access to x2apic MSRs

The x2apic specification reserves the entire MSR range 0x800-0xbff, while only
the first 0x3f MSRs have defined purposes.  All reserved MSRs in this region
are architecturally required to raise #GP faults upon access.

Xen used to pass this entire range to hvm_x2apic_msr_{read,write}(), but the
range was restricted somewhat by XSA-108 (c/s 61fdda7ac) to prevent guests
being able to read pages adjacent to the domheap page backing the vlapic->regs

While removing the vulnerability, a side effect of XSA-108 was that the MSR
range 0x900-0xbff fell through the switch statement and ends up reading the
hosts x2apic range. This behaviour is a problem in general, but specifically
it turns out that MSRs 0xa00-0xa02 are implemented (but undocumented) on
certain SandyBridge and IvyBridge systems.

Experimentally, no operating system in XenServer's test suite (including all
versions of Windows currently supported by Microsoft) ever peek at these MSRs,
even on hosts where some of them are implemented.

This patch undoes the fix for XSA-108 (c/s 61fdda7ac), returning the primary
bounds check to the entire specified range.  hvm_x2apic_msr_write() was always
safe, as it is whitelist based.  hvm_x2apic_msr_read() changes to a whitelist
approach, which avoids the vulnerability, and provides a more architecturally
accurate emulation of the reserved MSRs (which would previously read as 0
rather than fault).

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <JBeulich@xxxxxxxx>
Acked-by: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Matt Wilson <msw@xxxxxxxxx>
CC: Yang Zhang <yang.z.zhang@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
CC: Feng Wu <feng.wu@xxxxxxxxx>
CC: Eddie Dong <eddie.dong@xxxxxxxxx>
CC: Donald Dugger <donald.d.dugger@xxxxxxxxx>

 * Move vlapic_read_aligned() into the switch statement for better
   uninitalised variable checking.
 * XenRT tests happy with this version of the patch.
 * Non RFC.  Clarifications to wording of the commit message.
 * Switch to whitelist based approach.
 xen/arch/x86/hvm/hvm.c    |    4 ++--
 xen/arch/x86/hvm/vlapic.c |   27 +++++++++++++++++++++++----
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index f0e1edc..559b769 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4355,7 +4355,7 @@ int hvm_msr_read_intercept(unsigned int msr, uint64_t 
         *msr_content = vcpu_vlapic(v)->hw.apic_base_msr;
-    case MSR_IA32_APICBASE_MSR ... MSR_IA32_APICBASE_MSR + 0xff:
+    case MSR_IA32_APICBASE_MSR ... MSR_IA32_APICBASE_MSR + 0x3ff:
         if ( hvm_x2apic_msr_read(v, msr, msr_content) )
             goto gp_fault;
@@ -4482,7 +4482,7 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t 
         vlapic_tdt_msr_set(vcpu_vlapic(v), msr_content);
-    case MSR_IA32_APICBASE_MSR ... MSR_IA32_APICBASE_MSR + 0xff:
+    case MSR_IA32_APICBASE_MSR ... MSR_IA32_APICBASE_MSR + 0x3ff:
         if ( hvm_x2apic_msr_write(v, msr, msr_content) )
             goto gp_fault;
diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index 089d13f..2f09713 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -649,16 +649,35 @@ int hvm_x2apic_msr_read(struct vcpu *v, unsigned int msr, 
uint64_t *msr_content)
     if ( !vlapic_x2apic_mode(vlapic) )
         return X86EMUL_UNHANDLEABLE;
-    vlapic_read_aligned(vlapic, offset, &low);
     switch ( offset )
     case APIC_ICR:
         vlapic_read_aligned(vlapic, APIC_ICR2, &high);
+        /* Fallthrough. */
+    case APIC_ID:
+    case APIC_LVR:
+    case APIC_TASKPRI:
+    case APIC_PROCPRI:
+    case APIC_LDR:
+    case APIC_SPIV:
+    case APIC_ISR ... APIC_ISR + 0x70:
+    case APIC_TMR ... APIC_TMR + 0x70:
+    case APIC_IRR ... APIC_IRR + 0x70:
+    case APIC_ESR:
+    case APIC_CMCI:
+    case APIC_LVTT:
+    case APIC_LVTTHMR:
+    case APIC_LVTPC:
+    case APIC_LVT0:
+    case APIC_LVT1:
+    case APIC_LVTERR:
+    case APIC_TMICT:
+    case APIC_TMCCT:
+    case APIC_TDCR:
+        vlapic_read_aligned(vlapic, offset, &low);
-    case APIC_EOI:
-    case APIC_ICR2:
-    case APIC_SELF_IPI:
+    default:
         return X86EMUL_UNHANDLEABLE;

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