xen/gunzip: Allow perform_gunzip() to be called multiple times

Currently perform_gunzip() can only be called once because the
the internal state (e.g allocate) is not fully re-initialized.

This works fine if you are only booting dom0. But this will break when
booting multiple using the dom0less that uses compressed kernel images.

This can be resolved by re-initializing bytes_out, malloc_ptr,
malloc_count every time perform_gunzip() is called.

Note the latter is only re-initialized for hardening purpose as there is
no guarantee that every malloc() are followed by free() (It should in
theory!).

Take the opportunity to check the return of alloc_heap_pages() to return
an error rather than dereferencing a NULL pointer later on failure.

Reported-by: Charles Chiou <cchiou@ambarella.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/gunzip.c

@@ -114,11 +114,16 @@ __init int perform_gunzip(char *output, char *image, unsigned long image_len)
     window = (unsigned char *)output;
 
     free_mem_ptr = (unsigned long)alloc_xenheap_pages(HEAPORDER, 0);
+    if ( !free_mem_ptr )
+        return -ENOMEM;
+
     free_mem_end_ptr = free_mem_ptr + (PAGE_SIZE << HEAPORDER);
+    init_allocator();
 
     inbuf = (unsigned char *)image;
     insize = image_len;
     inptr = 0;
+    bytes_out = 0;
 
     makecrc();
 

xen/common/inflate.c

@@ -238,6 +238,12 @@ STATIC const ush mask_bits[] = {
 static unsigned long INITDATA malloc_ptr;
 static int INITDATA malloc_count;
 
+static void init_allocator(void)
+{
+    malloc_ptr = free_mem_ptr;
+    malloc_count = 0;
+}
+
 static void *INIT malloc(int size)
 {
     void *p;