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x86/HVM: confine internally handled MMIO to solitary regions

While it is generally wrong to cross region boundaries when dealing
with MMIO accesses of repeated string instructions (currently only
MOVS) as that would do things a guest doesn't expect (leaving aside
that none of these regions would normally be accessed with repeated
string instructions in the first place), this is even more of a problem
for all virtual MSI-X page accesses (both msixtbl_{read,write}() can be
made dereference NULL "entry" pointers this way) as well as undersized
(1- or 2-byte) LAPIC writes (causing vlapic_read_aligned() to access
space beyond the one memory page set up for holding LAPIC register
values).

Since those functions validly assume to be called only with addresses
their respective checking functions indicated to be okay, it is generic
code that needs to be fixed to clip the repetition count.

To be on the safe side (and consistent), also do the same for buffered
I/O intercepts, even if their only client (stdvga) doesn't put the
hypervisor at risk (i.e. "only" guest misbehavior would result).

This is CVE-2014-8867 / XSA-112.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

--- a/xen/arch/x86/hvm/intercept.c
+++ b/xen/arch/x86/hvm/intercept.c
@@ -131,11 +131,24 @@ int hvm_mmio_intercept(ioreq_t *p)
     int i;
 
     for ( i = 0; i < HVM_MMIO_HANDLER_NR; i++ )
-        if ( hvm_mmio_handlers[i]->check_handler(v, p->addr) )
+    {
+        hvm_mmio_check_t check_handler =
+            hvm_mmio_handlers[i]->check_handler;
+
+        if ( check_handler(v, p->addr) )
+        {
+            if ( unlikely(p->count > 1) &&
+                 !check_handler(v, unlikely(p->df)
+                                   ? p->addr - (p->count - 1LL) * p->size
+                                   : p->addr + (p->count - 1LL) * p->size) )
+                p->count = 1;
+
             return hvm_mmio_access(
                 v, p,
                 hvm_mmio_handlers[i]->read_handler,
                 hvm_mmio_handlers[i]->write_handler);
+        }
+    }
 
     return X86EMUL_UNHANDLEABLE;
 }
@@ -243,6 +256,13 @@ int hvm_io_intercept(ioreq_t *p, int typ
             if ( type == HVM_PORTIO )
                 return process_portio_intercept(
                     handler->hdl_list[i].action.portio, p);
+
+            if ( unlikely(p->count > 1) &&
+                 (unlikely(p->df)
+                  ? p->addr - (p->count - 1LL) * p->size < addr
+                  : p->addr + p->count * 1LL * p->size - 1 >= addr + size) )
+                p->count = 1;
+
             return handler->hdl_list[i].action.mmio(p);
         }
     }
--- a/xen/arch/x86/hvm/vmsi.c
+++ b/xen/arch/x86/hvm/vmsi.c
@@ -236,6 +236,8 @@ static int msixtbl_read(
     rcu_read_lock(&msixtbl_rcu_lock);
 
     entry = msixtbl_find_entry(v, address);
+    if ( !entry )
+        goto out;
     offset = address & (PCI_MSIX_ENTRY_SIZE - 1);
 
     if ( offset != PCI_MSIX_ENTRY_VECTOR_CTRL_OFFSET )
@@ -278,6 +280,8 @@ static int msixtbl_write(struct vcpu *v,
     rcu_read_lock(&msixtbl_rcu_lock);
 
     entry = msixtbl_find_entry(v, address);
+    if ( !entry )
+        goto out;
     nr_entry = (address - entry->gtable) / PCI_MSIX_ENTRY_SIZE;
 
     offset = address & (PCI_MSIX_ENTRY_SIZE - 1);