oslib: allocate PROT_NONE pages on top of RAM

This inserts a read and write protected page between RAM and QEMU
memory. This makes it harder to exploit QEMU bugs resulting from buffer
overflows in devices using variants of cpu_physical_memory_map,
dma_memory_map etc.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

util/oslib-posix.c

@@ -128,7 +128,7 @@ void *qemu_memalign(size_t alignment, size_t size)
 void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
 {
     size_t align = QEMU_VMALLOC_ALIGN;
-    size_t total = size + align - getpagesize();
+    size_t total = size + align;
     void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
     size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
     void *ptr1;
@@ -154,8 +154,8 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment)
     if (offset > 0) {
         munmap(ptr - offset, offset);
     }
-    if (total > size) {
-        munmap(ptr + size, total - size);
+    if (total > size + getpagesize()) {
+        munmap(ptr + size + getpagesize(), total - size - getpagesize());
     }
 
     trace_qemu_anon_ram_alloc(size, ptr);
@@ -172,7 +172,7 @@ void qemu_anon_ram_free(void *ptr, size_t size)
 {
     trace_qemu_anon_ram_free(ptr, size);
     if (ptr) {
-        munmap(ptr, size);
+        munmap(ptr, size + getpagesize());
     }
 }