[PATCH v2] libsepol/cil: fix NULL pointer dereference when parsing an improper integer

[Nicolas Iooss <nicolas.iooss@xxxxxxx>]

OSS-Fuzz found a NULL pointer dereference when the CIL compiler tries to
compile a policy with an invalid integer:

    $ echo '(ioportcon(2())n)' > tmp.cil
    $ secilc tmp.cil
    Segmentation fault (core dumped)

This is because strtol() is called with a NULL pointer, in
cil_fill_integer().

Fix this by checking that int_node->data is not NULL. While at it, use
strtoul() instead of strtol() to parse an unsigned integer.

When using "val > UINT32_MAX" with "unsigned long val;", it is expected
that some compilers emit a warning when the size of "unsigned long" is
32 bits. In theory gcc could be such a compiler (with warning
-Wtype-limits, which is included in -Wextra). Nevertheless this is
currently broken, according to
https://gcc.gnu.org/pipermail/gcc-help/2021-January/139755.html and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89126 (this bug was
opened in January 2019).

In order to prevent this warning from appearing, introduce some
preprocessor macros around the bound check.

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28456
Signed-off-by: Nicolas Iooss <nicolas.iooss@xxxxxxx>
---
 libsepol/cil/src/cil_build_ast.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/libsepol/cil/src/cil_build_ast.c b/libsepol/cil/src/cil_build_ast.c
index be10d61b1314..02481558ad11 100644
--- a/libsepol/cil/src/cil_build_ast.c
+++ b/libsepol/cil/src/cil_build_ast.c
@@ -5570,19 +5570,27 @@ int cil_fill_integer(struct cil_tree_node *int_node, uint32_t *integer, int base
 {
 	int rc = SEPOL_ERR;
 	char *endptr = NULL;
-	int val;
+	unsigned long val;
 
-	if (int_node == NULL || integer == NULL) {
+	if (int_node == NULL || int_node->data == NULL || integer == NULL) {
 		goto exit;
 	}
 
 	errno = 0;
-	val = strtol(int_node->data, &endptr, base);
+	val = strtoul(int_node->data, &endptr, base);
 	if (errno != 0 || endptr == int_node->data || *endptr != '\0') {
 		rc = SEPOL_ERR;
 		goto exit;
 	}
 
+	/* Ensure that the value fits a 32-bit integer without triggering -Wtype-limits */
+#if ULONG_MAX > UINT32_MAX
+	if (val > UINT32_MAX) {
+		rc = SEPOL_ERR;
+		goto exit;
+	}
+#endif
+
 	*integer = val;
 
 	return SEPOL_OK;
@@ -5598,7 +5606,7 @@ int cil_fill_integer64(struct cil_tree_node *int_node, uint64_t *integer, int ba
 	char *endptr = NULL;
 	uint64_t val;
 
-	if (int_node == NULL || integer == NULL) {
+	if (int_node == NULL || int_node->data == NULL || integer == NULL) {
 		goto exit;
 	}
 
-- 
2.30.0