VMX: flush cache when vmentry back to UC guest

This patch flush cache when vmentry back to UC guest, to prevent
cache polluted by hypervisor access guest memory during UC mode.

The elegant way to do this is, simply add wbinvd just before vmentry.
However, currently wbinvd before vmentry will mysteriously trigger
lapic timer interrupt storm, hung booting stage for 10s ~ 60s. We still
didn't dig out the root cause of interrupt storm, so currently this
patch add flag indicating hypervisor access UC guest memory to prevent
interrupt storm -- though it still leaves aspects un-addressed, i.e.
speculative reads, and multi-vCPU issues, etc.

Whenever the interrupt storm got root caused and fixed, the protection
flag can be removed -- that would be final clean and elegant approach
dealing with cache flushing before vmentry.

This is CVE-2013-2212 / XSA-60.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>

xen/arch/x86/hvm/hvm.c

@@ -2477,6 +2477,9 @@ static enum hvm_copy_result __hvm_copy(
         return HVMCOPY_unhandleable;
 #endif
 
+    if ( unlikely(curr->arch.hvm_vcpu.cache_mode == NO_FILL_CACHE_MODE) )
+        curr->arch.hvm_vcpu.hypervisor_access_uc_hvm_memory = 1;
+
     while ( todo > 0 )
     {
         count = min_t(int, PAGE_SIZE - (addr & ~PAGE_MASK), todo);
@@ -2588,6 +2591,9 @@ static enum hvm_copy_result __hvm_clear(paddr_t addr, int size)
         return HVMCOPY_unhandleable;
 #endif
 
+    if ( unlikely(curr->arch.hvm_vcpu.cache_mode == NO_FILL_CACHE_MODE) )
+        curr->arch.hvm_vcpu.hypervisor_access_uc_hvm_memory = 1;
+
     while ( todo > 0 )
     {
         count = min_t(int, PAGE_SIZE - (addr & ~PAGE_MASK), todo);

xen/arch/x86/hvm/vmx/vmx.c

@@ -2974,6 +2974,13 @@ void vmx_vmenter_helper(const struct cpu_user_regs *regs)
     struct hvm_vcpu_asid *p_asid;
     bool_t need_flush;
 
+    /* In case hypervisor access hvm memory when guest uc mode */
+    if ( unlikely(curr->arch.hvm_vcpu.hypervisor_access_uc_hvm_memory) )
+    {
+        curr->arch.hvm_vcpu.hypervisor_access_uc_hvm_memory = 0;
+        wbinvd();
+    }
+
     if ( !cpu_has_vmx_vpid )
         goto out;
     if ( nestedhvm_vcpu_in_guestmode(curr) )

xen/include/asm-x86/hvm/vcpu.h

@@ -167,6 +167,7 @@ struct hvm_vcpu {
 
     /* Which cache mode is this VCPU in (CR0:CD/NW)? */
     u8                  cache_mode;
+    bool_t              hypervisor_access_uc_hvm_memory;
 
     struct hvm_vcpu_io  hvm_io;