liblzma: lzma_filters_copy: Keep dest[] unmodified if an error occurs.

lzma_stream_encoder() and lzma_stream_encoder_mt() always assumed
this. Before this patch, failing lzma_filters_copy() could result
in free(invalid_pointer) or invalid memory reads in stream_encoder.c
or stream_encoder_mt.c.

To trigger this, allocating memory for a filter options structure
has to fail. These are tiny allocations so in practice they very
rarely fail.

Certain badness in the filter chain array could also make
lzma_filters_copy() fail but both stream_encoder.c and
stream_encoder_mt.c validate the filter chain before
trying to copy it, so the crash cannot occur this way.

src/liblzma/api/lzma/filter.h

@@ -108,7 +108,9 @@ extern LZMA_API(lzma_bool) lzma_filter_decoder_is_supported(lzma_vli id)
  * need to be initialized by the caller in any way.
  *
  * If an error occurs, memory possibly already allocated by this function
- * is always freed.
+ * is always freed. liblzma versions older than 5.2.7 may modify the dest
+ * array and leave its contents in an undefined state if an error occurs.
+ * liblzma 5.2.7 and newer only modify the dest array when returning LZMA_OK.
  *
  * \return      - LZMA_OK
  *              - LZMA_MEM_ERROR

src/liblzma/common/filter_common.c

@@ -122,12 +122,16 @@ static const struct {
 
 
 extern LZMA_API(lzma_ret)
-lzma_filters_copy(const lzma_filter *src, lzma_filter *dest,
+lzma_filters_copy(const lzma_filter *src, lzma_filter *real_dest,
 		const lzma_allocator *allocator)
 {
-	if (src == NULL || dest == NULL)
+	if (src == NULL || real_dest == NULL)
 		return LZMA_PROG_ERROR;
 
+	// Use a temporary destination so that the real destination
+	// will never be modied if an error occurs.
+	lzma_filter dest[LZMA_FILTERS_MAX + 1];
+
 	lzma_ret ret;
 	size_t i;
 	for (i = 0; src[i].id != LZMA_VLI_UNKNOWN; ++i) {
@@ -173,18 +177,20 @@ lzma_filters_copy(const lzma_filter *src, lzma_filter *dest,
 	}
 
 	// Terminate the filter array.
-	assert(i <= LZMA_FILTERS_MAX + 1);
+	assert(i < LZMA_FILTERS_MAX + 1);
 	dest[i].id = LZMA_VLI_UNKNOWN;
 	dest[i].options = NULL;
 
+	// Copy it to the caller-supplied array now that we know that
+	// no errors occurred.
+	memcpy(real_dest, dest, (i + 1) * sizeof(lzma_filter));
+
 	return LZMA_OK;
 
 error:
 	// Free the options which we have already allocated.
-	while (i-- > 0) {
+	while (i-- > 0)
 		lzma_free(dest[i].options, allocator);
-		dest[i].options = NULL;
-	}
 
 	return ret;
 }