pack-bitmap: bounds-check size of cache extension

A .bitmap file may have a "name hash cache" extension, which puts a
sequence of uint32_t values (one per object) at the end of the file.
When we see a flag indicating this extension, we blindly subtract the
appropriate number of bytes from our available length. However, if the
.bitmap file is too short, we'll underflow our length variable and wrap
around, thinking we have a very large length. This can lead to reading
out-of-bounds bytes while loading individual ewah bitmaps.

We can fix this by checking the number of available bytes when we parse
the header. The existing "truncated bitmap" test is now split into two
tests: one where we don't have this extension at all (and hence actually
do try to read a truncated ewah bitmap) and one where we realize
up-front that we can't even fit in the cache structure. We'll check
stderr in each case to make sure we hit the error we're expecting.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/pack-bitmap.c b/pack-bitmap.c
index fe5647e72e..074d9ac8f2 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -153,14 +153,18 @@ static int load_bitmap_header(struct bitmap_index *index)
 	/* Parse known bitmap format options */
 	{
 		uint32_t flags = ntohs(header->options);
+		size_t cache_size = st_mult(index->pack->num_objects, sizeof(uint32_t));
+		unsigned char *index_end = index->map + index->map_size - the_hash_algo->rawsz;
 
 		if ((flags & BITMAP_OPT_FULL_DAG) == 0)
 			return error("Unsupported options for bitmap index file "
 				"(Git requires BITMAP_OPT_FULL_DAG)");
 
 		if (flags & BITMAP_OPT_HASH_CACHE) {
-			unsigned char *end = index->map + index->map_size - the_hash_algo->rawsz;
-			index->hashes = ((uint32_t *)end) - index->pack->num_objects;
+			if (cache_size > index_end - index->map - header_size)
+				return error("corrupted bitmap index file (too short to fit hash cache)");
+			index->hashes = (void *)(index_end - cache_size);
+			index_end -= cache_size;
 		}
 	}