x86/HVM: correct MMIO emulation cache bounds check

To avoid overrunning the internal buffer we need to take the offset into
the buffer into account.

Fixes: d95da91fb4 ("x86/HVM: grow MMIO cache data size to 64 bytes")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: e5339bb689
master date: 2025-01-23 11:14:48 +0100

xen/arch/x86/hvm/emulate.c

@@ -937,7 +937,7 @@ static int hvmemul_phys_mmio_access(
     }
 
     /* Accesses must not overflow the cache's buffer. */
-    if ( size > sizeof(cache->buffer) )
+    if ( offset + size > sizeof(cache->buffer) )
     {
         ASSERT_UNREACHABLE();
         return X86EMUL_UNHANDLEABLE;