Fuzzing and bugfixes for magicless-format decoding

CHANGELOG

@@ -9,6 +9,7 @@ lib: accept dictionaries with partial literal tables, by @terrelln
 lib: fix CCtx size estimation with external sequence producer, by @embg
 lib: fix corner case decoder behaviors, by @Cyan4973 and @aimuz
 lib: fix zdict prototype mismatch in static_only mode, by @ldv-alt
+lib: fix several bugs in magicless-format decoding, by @embg
 cli: add common compressed file types to `--exclude-compressed`` by @daniellerozenblit
 cli: fix mixing `-c` and `-o` commands with `--rm`, by @Cyan4973
 cli: fix erroneous exclusion of hidden files with `--output-dir-mirror` by @felixhandte

doc/decompressor_errata.md

@@ -125,3 +125,24 @@ The total `Block_Content` is `5` bytes, and `Last_Table_Offset` is `2`.
 See the compressor workaround code:
 
 https://github.com/facebook/zstd/blob/8814aa5bfa74f05a86e55e9d508da177a893ceeb/lib/compress/zstd_compress.c#L2667-L2682
+
+Magicless format
+----------------------
+
+**Last affected version**: v1.5.5
+
+**Affected decompressor component(s)**: Library & CLI
+
+**Produced by the reference compressor**: Yes (example: https://gist.github.com/embg/9940726094f4cf2cef162cffe9319232)
+
+**Example Frame**: `27 b5 2f fd 00 03 19 00 00 66 6f 6f 3f ba c4 59`
+
+v1.5.6 fixes several bugs in which the magicless-format decoder rejects valid frames.
+These include but are not limited to:
+* Valid frames that happen to begin with a legacy magic number (little-endian)
+* Valid frames that happen to begin with a skippable magic number (little-endian)
+
+If you are affected by this issue and cannot update to v1.5.6 or later, there is a
+workaround to recover affected data. Simply prepend the ZSTD magic number
+`0xFD2FB528` (little-endian) to your data and decompress using the standard-format
+decoder.

lib/decompress/zstd_decompress.c

@@ -1085,7 +1085,7 @@ size_t ZSTD_decompressMultiFrame(ZSTD_DCtx* dctx,
     while (srcSize >= ZSTD_startingInputLength(dctx->format)) {
 
 #if defined(ZSTD_LEGACY_SUPPORT) && (ZSTD_LEGACY_SUPPORT >= 1)
-        if (ZSTD_isLegacy(src, srcSize)) {
+        if (dctx->format == ZSTD_f_zstd1 && ZSTD_isLegacy(src, srcSize)) {
             size_t decodedSize;
             size_t const frameSize = ZSTD_findFrameCompressedSizeLegacy(src, srcSize);
             if (ZSTD_isError(frameSize)) return frameSize;
@@ -1115,7 +1115,7 @@ size_t ZSTD_decompressMultiFrame(ZSTD_DCtx* dctx,
         }
 #endif
 
-        if (srcSize >= 4) {
+        if (dctx->format == ZSTD_f_zstd1 && srcSize >= 4) {
             U32 const magicNumber = MEM_readLE32(src);
             DEBUGLOG(5, "reading magic number %08X", (unsigned)magicNumber);
             if ((magicNumber & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) {
@@ -1412,6 +1412,7 @@ size_t ZSTD_decompressContinue(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, c
     case ZSTDds_decodeSkippableHeader:
         assert(src != NULL);
         assert(srcSize <= ZSTD_SKIPPABLEHEADERSIZE);
+        assert(dctx->format != ZSTD_f_zstd1_magicless);
         ZSTD_memcpy(dctx->headerBuffer + (ZSTD_SKIPPABLEHEADERSIZE - srcSize), src, srcSize);   /* complete skippable header */
         dctx->expected = MEM_readLE32(dctx->headerBuffer + ZSTD_FRAMEIDSIZE);   /* note : dctx->expected can grow seriously large, beyond local buffer size */
         dctx->stage = ZSTDds_skipFrame;
@@ -2209,7 +2210,8 @@ size_t ZSTD_decompressStream(ZSTD_DStream* zds, ZSTD_outBuffer* output, ZSTD_inB
             DEBUGLOG(4, "Consume header");
             FORWARD_IF_ERROR(ZSTD_decompressBegin_usingDDict(zds, ZSTD_getDDict(zds)), "");
 
-            if ((MEM_readLE32(zds->headerBuffer) & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) {  /* skippable frame */
+            if (zds->format == ZSTD_f_zstd1
+                && (MEM_readLE32(zds->headerBuffer) & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) {  /* skippable frame */
                 zds->expected = MEM_readLE32(zds->headerBuffer + ZSTD_FRAMEIDSIZE);
                 zds->stage = ZSTDds_skipFrame;
             } else {

tests/fuzz/Makefile

@@ -124,7 +124,8 @@ FUZZ_TARGETS :=       \
 	sequence_compression_api \
 	seekable_roundtrip \
 	huf_round_trip \
-	huf_decompress
+	huf_decompress \
+	decompress_cross_format
 
 all: libregression.a $(FUZZ_TARGETS)
 
@@ -238,6 +239,9 @@ huf_round_trip: $(FUZZ_HEADERS) $(FUZZ_ROUND_TRIP_OBJ) rt_fuzz_huf_round_trip.o
 
 huf_decompress: $(FUZZ_HEADERS) $(FUZZ_DECOMPRESS_OBJ) d_fuzz_huf_decompress.o
 	$(CXX) $(FUZZ_TARGET_FLAGS) $(FUZZ_DECOMPRESS_OBJ) d_fuzz_huf_decompress.o $(LIB_FUZZING_ENGINE) -o $@
+
+decompress_cross_format: $(FUZZ_HEADERS) $(FUZZ_DECOMPRESS_OBJ) d_fuzz_decompress_cross_format.o
+	$(CXX) $(FUZZ_TARGET_FLAGS) $(FUZZ_DECOMPRESS_OBJ) d_fuzz_decompress_cross_format.o $(LIB_FUZZING_ENGINE) -o $@
 
 libregression.a: $(FUZZ_HEADERS) $(PRGDIR)/util.h $(PRGDIR)/util.c d_fuzz_regression_driver.o
 	$(AR) $(FUZZ_ARFLAGS) $@ d_fuzz_regression_driver.o

tests/fuzz/decompress_cross_format.c

@@ -0,0 +1,130 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+// This fuzz target validates decompression of magicless-format compressed data.
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include "fuzz_helpers.h"
+#define ZSTD_STATIC_LINKING_ONLY
+#include "zstd.h"
+#include "fuzz_data_producer.h"
+
+static ZSTD_DCtx *dctx = NULL;
+
+int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
+{
+    // Give a random portion of src data to the producer, to use for parameter generation.
+    // The rest will be interpreted as magicless compressed data.
+    FUZZ_dataProducer_t *producer = FUZZ_dataProducer_create(src, size);
+    size_t magiclessSize = FUZZ_dataProducer_reserveDataPrefix(producer);
+    const void* const magiclessSrc = src;
+    size_t const dstSize = FUZZ_dataProducer_uint32Range(producer, 0, 10 * size);
+    void* const standardDst = FUZZ_malloc(dstSize);
+    void* const magiclessDst = FUZZ_malloc(dstSize);
+
+    // Create standard-format src from magicless-format src
+    const uint32_t zstd_magic = ZSTD_MAGICNUMBER;
+    size_t standardSize = sizeof(zstd_magic) + magiclessSize;
+    void* const standardSrc = FUZZ_malloc(standardSize);
+    memcpy(standardSrc, &zstd_magic, sizeof(zstd_magic)); // assume fuzzing on little-endian machine
+    memcpy(standardSrc + sizeof(zstd_magic), magiclessSrc, magiclessSize);
+
+    // Truncate to a single frame
+    {
+        const size_t standardFrameCompressedSize = ZSTD_findFrameCompressedSize(standardSrc, standardSize);
+        if (ZSTD_isError(standardFrameCompressedSize)) {
+            goto cleanup_and_return;
+        }
+        standardSize = standardFrameCompressedSize;
+        magiclessSize = standardFrameCompressedSize - sizeof(zstd_magic);
+    }
+
+    // Create DCtx if needed
+    if (!dctx) {
+        dctx = ZSTD_createDCtx();
+        FUZZ_ASSERT(dctx);
+    }
+
+    // Test one-shot decompression
+    {
+        FUZZ_ZASSERT(ZSTD_DCtx_reset(dctx, ZSTD_reset_session_and_parameters));
+        FUZZ_ZASSERT(ZSTD_DCtx_setParameter(dctx, ZSTD_d_format, ZSTD_f_zstd1));
+        const size_t standardRet = ZSTD_decompressDCtx(
+                                        dctx, standardDst, dstSize, standardSrc, standardSize);
+
+        FUZZ_ZASSERT(ZSTD_DCtx_reset(dctx, ZSTD_reset_session_and_parameters));
+        FUZZ_ZASSERT(ZSTD_DCtx_setParameter(dctx, ZSTD_d_format, ZSTD_f_zstd1_magicless));
+        const size_t magiclessRet = ZSTD_decompressDCtx(
+                                        dctx, magiclessDst, dstSize, magiclessSrc, magiclessSize);
+
+        // Standard accepts => magicless should accept
+        if (!ZSTD_isError(standardRet)) FUZZ_ZASSERT(magiclessRet);
+
+        // Magicless accepts => standard should accept
+        // NOTE: this is nice-to-have, please disable this check if it is difficult to satisfy.
+        if (!ZSTD_isError(magiclessRet)) FUZZ_ZASSERT(standardRet);
+
+        // If both accept, decompressed size and data should match
+        if (!ZSTD_isError(standardRet) && !ZSTD_isError(magiclessRet)) {
+            FUZZ_ASSERT(standardRet == magiclessRet);
+            if (standardRet > 0) {
+                FUZZ_ASSERT(
+                    memcmp(standardDst, magiclessDst, standardRet) == 0
+                );
+            }
+        }
+    }
+
+    // Test streaming decompression
+    {
+        ZSTD_inBuffer standardIn = { standardSrc, standardSize, 0 };
+        ZSTD_inBuffer magiclessIn = { magiclessSrc, magiclessSize, 0 };
+        ZSTD_outBuffer standardOut = { standardDst, dstSize, 0 };
+        ZSTD_outBuffer magiclessOut = { magiclessDst, dstSize, 0 };
+
+        FUZZ_ZASSERT(ZSTD_DCtx_reset(dctx, ZSTD_reset_session_and_parameters));
+        FUZZ_ZASSERT(ZSTD_DCtx_setParameter(dctx, ZSTD_d_format, ZSTD_f_zstd1));
+        const size_t standardRet = ZSTD_decompressStream(dctx, &standardOut, &standardIn);
+
+        FUZZ_ZASSERT(ZSTD_DCtx_reset(dctx, ZSTD_reset_session_and_parameters));
+        FUZZ_ZASSERT(ZSTD_DCtx_setParameter(dctx, ZSTD_d_format, ZSTD_f_zstd1_magicless));
+        const size_t magiclessRet = ZSTD_decompressStream(dctx, &magiclessOut, &magiclessIn);
+
+        // Standard accepts => magicless should accept
+        if (standardRet == 0) FUZZ_ASSERT(magiclessRet == 0);
+
+        // Magicless accepts => standard should accept
+        // NOTE: this is nice-to-have, please disable this check if it is difficult to satisfy.
+        if (magiclessRet == 0) FUZZ_ASSERT(standardRet == 0);
+
+        // If both accept, decompressed size and data should match
+        if (standardRet == 0 && magiclessRet == 0) {
+            FUZZ_ASSERT(standardOut.pos == magiclessOut.pos);
+            if (standardOut.pos > 0) {
+                FUZZ_ASSERT(
+                    memcmp(standardOut.dst, magiclessOut.dst, standardOut.pos) == 0
+                );
+            }
+        }
+    }
+
+cleanup_and_return:
+#ifndef STATEFUL_FUZZING
+    ZSTD_freeDCtx(dctx); dctx = NULL;
+#endif
+    free(standardSrc);
+    free(standardDst);
+    free(magiclessDst);
+    FUZZ_dataProducer_free(producer);
+    return 0;
+}

tests/fuzz/fuzz.py

@@ -65,6 +65,7 @@ def __init__(self, input_type, frame_type=FrameType.ZSTD):
     'seekable_roundtrip': TargetInfo(InputType.RAW_DATA),
     'huf_round_trip': TargetInfo(InputType.RAW_DATA),
     'huf_decompress': TargetInfo(InputType.RAW_DATA),
+    'decompress_cross_format': TargetInfo(InputType.RAW_DATA),
 }
 TARGETS = list(TARGET_INFO.keys())
 ALL_TARGETS = TARGETS + ['all']