Check null stack to prevent heap-buffer-overflow

This patch adds a new macro STACK_NULL to check if given stack was initialized,
in order to fix #298, which is CVE-2024-35329.

The root cause is stack(document->nodes) was used before initialized, so check stack
before push.

According to the poc in [1], building it with
`gcc poc.c -o poc -lyaml -fsanitize=address`

Before this patch, the output is:
[root@test yaml-0.2.5]# ./poc
heap-buffer-overflow on libyaml/src/api.c:1274:10

=================================================================
==3867981==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 64 byte(s) in 1 object(s) allocated from:
    #0 0x7f571f6af1a7 in __interceptor_malloc (/usr/lib64/libasan.so.6+0xaf1a7)
    #1 0x7f5720127ac9 in yaml_document_add_sequence /root/libxml/yaml-0.2.5/src/api.c:1271

Direct leak of 22 byte(s) in 1 object(s) allocated from:
    #0 0x7f571f659707 in strdup (/usr/lib64/libasan.so.6+0x59707)
    #1 0x7f5720127ab7 in yaml_document_add_sequence /root/libxml/yaml-0.2.5/src/api.c:1268

Direct leak of 1 byte(s) in 1 object(s) allocated from:
    #0 0x7f571f6af1a7 in __interceptor_malloc (/usr/lib64/libasan.so.6+0xaf1a7)
    #1 0x7f5720125762 in yaml_stack_extend /root/libxml/yaml-0.2.5/src/api.c:126

SUMMARY: AddressSanitizer: 87 byte(s) leaked in 3 allocation(s).

After this patch, there are no memory leaks warnnings.

[1] https://drive.google.com/file/d/1xgQ9hJ7Sn5RVEsdMGvIy0s3b_bg3Wyk-/view?usp=sharing

Signed-off-by: Zhao Mengmeng <[email protected]>

src/api.c

@@ -1206,6 +1206,7 @@ yaml_document_add_scalar(yaml_document_t *document,
 
     assert(document);   /* Non-NULL document object is expected. */
     assert(value);      /* Non-NULL value is expected. */
+    if (STACK_NULL(&context, document->nodes)) goto error;
 
     if (!tag) {
         tag = (yaml_char_t *)YAML_DEFAULT_SCALAR_TAG;
@@ -1258,6 +1259,7 @@ yaml_document_add_sequence(yaml_document_t *document,
     yaml_node_t node;
 
     assert(document);   /* Non-NULL document object is expected. */
+    if (STACK_NULL(&context, document->nodes)) goto error;
 
     if (!tag) {
         tag = (yaml_char_t *)YAML_DEFAULT_SEQUENCE_TAG;
@@ -1303,6 +1305,7 @@ yaml_document_add_mapping(yaml_document_t *document,
     yaml_node_t node;
 
     assert(document);   /* Non-NULL document object is expected. */
+    if (STACK_NULL(&context, document->nodes)) goto error;
 
     if (!tag) {
         tag = (yaml_char_t *)YAML_DEFAULT_MAPPING_TAG;

src/yaml_private.h

@@ -420,6 +420,11 @@ yaml_stack_extend(void **start, void **top, void **end);
 YAML_DECLARE(int)
 yaml_queue_extend(void **start, void **head, void **tail, void **end);
 
+#define STACK_NULL(context,stack)                                               \
+  (((stack).start != NULL) ? 0 :                                                \
+        ((context)->error = YAML_MEMORY_ERROR,                                  \
+         1))
+
 #define STACK_INIT(context,stack,type)                                     \
   (((stack).start = (type)yaml_malloc(INITIAL_STACK_SIZE*sizeof(*(stack).start))) ? \
         ((stack).top = (stack).start,                                           \