asn1: Reject OIDs with too large sub-identifiers when converting to string

The shift would overflow the value which could produce garbage output
that might get interpreted as real OIDs (in case strings are compared).

This limit allows OID sub-identifiers to consist of at most 4 bytes,
which should be enough for any real-world OIDs (it's also the maximum we
used in tests so far).

Fixes: f813069e89 ("fixed asn1_oid_to_string() conversion")

src/libstrongswan/asn1/asn1.c

@@ -203,7 +203,7 @@ char *asn1_oid_to_string(chunk_t oid)
 	size_t len = 64;
 	char buf[len], *pos = buf;
 	int written;
-	u_int val;
+	uint32_t val;
 
 	if (!oid.len)
 	{
@@ -222,7 +222,11 @@ char *asn1_oid_to_string(chunk_t oid)
 
 	while (oid.len)
 	{
-		val = (val << 7) + (u_int)(oid.ptr[0] & 0x7f);
+		if (val > (UINT32_MAX >> 7))
+		{
+			return NULL;
+		}
+		val = (val << 7) + (uint32_t)(oid.ptr[0] & 0x7f);
 
 		if (oid.ptr[0] < 128)
 		{

src/libstrongswan/tests/suites/test_asn1.c

@@ -253,6 +253,7 @@ START_TEST(test_asn1_oid_to_string)
 			chunk_from_chars( 0x0a, 0x64, 0x87, 0x68, 0xce, 0x10, 0x86, 0x8d,
 			0x20, 0xbd, 0x84, 0x40, 0x84, 0xe2, 0xad, 0x00,
 			0xaf, 0xd7, 0xc2, 0x00, 0xff, 0xff, 0xff, 0x7f) },
+		{ NULL, chunk_from_chars( 0x00, 0xff, 0xff, 0xff, 0xff, 0x7f) },
 		{ NULL, chunk_from_chars(
 			0x0a, 0x02, 0x64, 0x87, 0x68, 0xce, 0x10, 0x86, 0x8d, 0x20,
 			0xbd, 0x84, 0x40, 0x84, 0xe2, 0xad, 0x00, 0xaf, 0xd7, 0xc2, 0x00,