check snappy block length before crc trailer in decode_snappy

lang/c/src/codec.c

@@ -142,6 +142,11 @@ static int decode_snappy(avro_codec_t c, void * data, int64_t len)
         uint32_t crc;
         size_t outlen;
 
+        if (len < 4) {
+                avro_set_error("Snappy block is too small to contain a CRC32 checksum");
+                return 1;
+        }
+
         if (snappy_uncompressed_length((const char*)data, len-4, &outlen) != SNAPPY_OK) {
 		avro_set_error("Uncompressed length error in snappy");
 		return 1;

lang/c/tests/CMakeLists.txt

@@ -88,3 +88,4 @@ add_avro_test_checkmem(test_avro_1691)
 add_avro_test_checkmem(test_avro_1906)
 add_avro_test_checkmem(test_avro_1904)
 add_avro_test_checkmem(test_avro_4246)
+add_avro_test_checkmem(test_avro_3807)

lang/c/tests/test_avro_3807.c

@@ -0,0 +1,87 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied.  See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+#include "avro.h"
+#include "avro_private.h"
+#include "codec.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/*
+ * A snappy block carries a four byte CRC32 trailer, so anything shorter
+ * than that cannot be a valid block. The length comes straight from the
+ * container file and decode_snappy used to compute len - 4 before checking
+ * it, which underflows for blocks of 0 to 3 bytes and reads out of bounds.
+ * Make sure those undersized blocks are rejected rather than decoded.
+ */
+
+int main(void)
+{
+#ifdef SNAPPY_CODEC
+	int i;
+
+	avro_codec_t codec = (avro_codec_t) avro_new(struct avro_codec_t_);
+	if (codec == NULL) {
+		fprintf(stderr, "Cannot allocate codec\n");
+		return EXIT_FAILURE;
+	}
+
+	if (avro_codec(codec, "snappy") != 0) {
+		fprintf(stderr, "Cannot create snappy codec: %s\n",
+			avro_strerror());
+		avro_freet(struct avro_codec_t_, codec);
+		return EXIT_FAILURE;
+	}
+
+	/*
+	 * Use an exact sized heap buffer for every length so a checker such as
+	 * valgrind catches any read past it. The bytes have the varint
+	 * continuation bit set so that, without the guard, the underflowed
+	 * len-4 lets snappy keep reading the length prefix off the end of the
+	 * block.
+	 */
+	for (i = 0; i < 4; i++) {
+		size_t size = (i == 0) ? 1 : (size_t) i;
+		char *buf = (char *) avro_malloc(size);
+		if (buf == NULL) {
+			avro_codec_reset(codec);
+			avro_freet(struct avro_codec_t_, codec);
+			return EXIT_FAILURE;
+		}
+		memset(buf, 0xff, size);
+
+		if (avro_codec_decode(codec, buf, i) == 0) {
+			fprintf(stderr,
+				"snappy block of %d bytes should be rejected\n",
+				i);
+			avro_free(buf, size);
+			avro_codec_reset(codec);
+			avro_freet(struct avro_codec_t_, codec);
+			return EXIT_FAILURE;
+		}
+
+		avro_free(buf, size);
+	}
+
+	avro_codec_reset(codec);
+	avro_freet(struct avro_codec_t_, codec);
+#else
+	fprintf(stderr, "Snappy codec not available, skipping\n");
+#endif
+	return EXIT_SUCCESS;
+}