Merge branch 'jk/pack-idx-corruption-safety' into maint

The code to read the pack data using the offsets stored in the pack
idx file has been made more carefully check the validity of the
data in the idx.

* jk/pack-idx-corruption-safety:
  sha1_file.c: mark strings for translation
  use_pack: handle signed off_t overflow
  nth_packed_object_offset: bounds-check extended offset
  t5313: test bounds-checks of corrupted/malicious pack/idx files

builtin/index-pack.c

@@ -1514,6 +1514,7 @@ static void read_v2_anomalous_offsets(struct packed_git *p,
 		if (!(off & 0x80000000))
 			continue;
 		off = off & 0x7fffffff;
+		check_pack_index_ptr(p, &idx2[off * 2]);
 		if (idx2[off * 2])
 			continue;
 		/*

cache.h

@@ -1367,6 +1367,16 @@ extern void free_pack_by_name(const char *);
 extern void clear_delta_base_cache(void);
 extern struct packed_git *add_packed_git(const char *path, size_t path_len, int local);
 
+/*
+ * Make sure that a pointer access into an mmap'd index file is within bounds,
+ * and can provide at least 8 bytes of data.
+ *
+ * Note that this is only necessary for variable-length segments of the file
+ * (like the 64-bit extended offset table), as we compare the size to the
+ * fixed-length parts when we open the file.
+ */
+extern void check_pack_index_ptr(const struct packed_git *p, const void *ptr);
+
 /*
  * Return the SHA-1 of the nth object within the specified packfile.
  * Open the index if it is not already open.  The return value points

sha1_file.c

@@ -1076,6 +1076,8 @@ unsigned char *use_pack(struct packed_git *p,
 		die("packfile %s cannot be accessed", p->pack_name);
 	if (offset > (p->pack_size - 20))
 		die("offset beyond end of packfile (truncated pack?)");
+	if (offset < 0)
+		die(_("offset before end of packfile (broken .idx?)"));
 
 	if (!win || !in_window(win, offset)) {
 		if (win)
@@ -2448,6 +2450,20 @@ const unsigned char *nth_packed_object_sha1(struct packed_git *p,
 	}
 }
 
+void check_pack_index_ptr(const struct packed_git *p, const void *vptr)
+{
+	const unsigned char *ptr = vptr;
+	const unsigned char *start = p->index_data;
+	const unsigned char *end = start + p->index_size;
+	if (ptr < start)
+		die(_("offset before start of pack index for %s (corrupt index?)"),
+		    p->pack_name);
+	/* No need to check for underflow; .idx files must be at least 8 bytes */
+	if (ptr >= end - 8)
+		die(_("offset beyond end of pack index for %s (truncated index?)"),
+		    p->pack_name);
+}
+
 off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
 {
 	const unsigned char *index = p->index_data;
@@ -2461,6 +2477,7 @@ off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
 		if (!(off & 0x80000000))
 			return off;
 		index += p->num_objects * 4 + (off & 0x7fffffff) * 8;
+		check_pack_index_ptr(p, index);
 		return (((uint64_t)ntohl(*((uint32_t *)(index + 0)))) << 32) |
 				   ntohl(*((uint32_t *)(index + 4)));
 	}

t/t5313-pack-bounds-checks.sh

@@ -0,0 +1,179 @@
+#!/bin/sh
+
+test_description='bounds-checking of access to mmapped on-disk file formats'
+. ./test-lib.sh
+
+clear_base () {
+	test_when_finished 'restore_base' &&
+	rm -f $base
+}
+
+restore_base () {
+	cp base-backup/* .git/objects/pack/
+}
+
+do_pack () {
+	pack_objects=$1; shift
+	sha1=$(
+		for i in $pack_objects
+		do
+			echo $i
+		done | git pack-objects "$@" .git/objects/pack/pack
+	) &&
+	pack=.git/objects/pack/pack-$sha1.pack &&
+	idx=.git/objects/pack/pack-$sha1.idx &&
+	chmod +w $pack $idx &&
+	test_when_finished 'rm -f "$pack" "$idx"'
+}
+
+munge () {
+	printf "$3" | dd of="$1" bs=1 conv=notrunc seek=$2
+}
+
+# Offset in a v2 .idx to its initial and extended offset tables. For an index
+# with "nr" objects, this is:
+#
+#   magic(4) + version(4) + fan-out(4*256) + sha1s(20*nr) + crc(4*nr),
+#
+# for the initial, and another ofs(4*nr) past that for the extended.
+#
+ofs_table () {
+	echo $((4 + 4 + 4*256 + 20*$1 + 4*$1))
+}
+extended_table () {
+	echo $(($(ofs_table "$1") + 4*$1))
+}
+
+test_expect_success 'set up base packfile and variables' '
+	# the hash of this content starts with ff, which
+	# makes some later computations much simpler
+	echo 74 >file &&
+	git add file &&
+	git commit -m base &&
+	git repack -ad &&
+	base=$(echo .git/objects/pack/*) &&
+	chmod +w $base &&
+	mkdir base-backup &&
+	cp $base base-backup/ &&
+	object=$(git rev-parse HEAD:file)
+'
+
+test_expect_success 'pack/index object count mismatch' '
+	do_pack $object &&
+	munge $pack 8 "\377\0\0\0" &&
+	clear_base &&
+
+	# We enumerate the objects from the completely-fine
+	# .idx, but notice later that the .pack is bogus
+	# and fail to show any data.
+	echo "$object missing" >expect &&
+	git cat-file --batch-all-objects --batch-check >actual &&
+	test_cmp expect actual &&
+
+	# ...and here fail to load the object (without segfaulting),
+	# but fallback to a good copy if available.
+	test_must_fail git cat-file blob $object &&
+	restore_base &&
+	git cat-file blob $object >actual &&
+	test_cmp file actual &&
+
+	# ...and make sure that index-pack --verify, which has its
+	# own reading routines, does not segfault.
+	test_must_fail git index-pack --verify $pack
+'
+
+test_expect_success 'matched bogus object count' '
+	do_pack $object &&
+	munge $pack 8 "\377\0\0\0" &&
+	munge $idx $((255 * 4)) "\377\0\0\0" &&
+	clear_base &&
+
+	# Unlike above, we should notice early that the .idx is totally
+	# bogus, and not even enumerate its contents.
+	>expect &&
+	git cat-file --batch-all-objects --batch-check >actual &&
+	test_cmp expect actual &&
+
+	# But as before, we can do the same object-access checks.
+	test_must_fail git cat-file blob $object &&
+	restore_base &&
+	git cat-file blob $object >actual &&
+	test_cmp file actual &&
+
+	test_must_fail git index-pack --verify $pack
+'
+
+# Note that we cannot check the fallback case for these
+# further .idx tests, as we notice the problem in functions
+# whose interface doesn't allow an error return (like use_pack()),
+# and thus we just die().
+#
+# There's also no point in doing enumeration tests, as
+# we are munging offsets here, which are about looking up
+# specific objects.
+
+test_expect_success 'bogus object offset (v1)' '
+	do_pack $object --index-version=1 &&
+	munge $idx $((4 * 256)) "\377\0\0\0" &&
+	clear_base &&
+	test_must_fail git cat-file blob $object &&
+	test_must_fail git index-pack --verify $pack
+'
+
+test_expect_success 'bogus object offset (v2, no msb)' '
+	do_pack $object --index-version=2 &&
+	munge $idx $(ofs_table 1) "\0\377\0\0" &&
+	clear_base &&
+	test_must_fail git cat-file blob $object &&
+	test_must_fail git index-pack --verify $pack
+'
+
+test_expect_success 'bogus offset into v2 extended table' '
+	do_pack $object --index-version=2 &&
+	munge $idx $(ofs_table 1) "\377\0\0\0" &&
+	clear_base &&
+	test_must_fail git cat-file blob $object &&
+	test_must_fail git index-pack --verify $pack
+'
+
+test_expect_success 'bogus offset inside v2 extended table' '
+	# We need two objects here, so we can plausibly require
+	# an extended table (if the first object were larger than 2^31).
+	do_pack "$object $(git rev-parse HEAD)" --index-version=2 &&
+
+	# We have to make extra room for the table, so we cannot
+	# just munge in place as usual.
+	{
+		dd if=$idx bs=1 count=$(($(ofs_table 2) + 4)) &&
+		printf "\200\0\0\0" &&
+		printf "\377\0\0\0\0\0\0\0" &&
+		dd if=$idx bs=1 skip=$(extended_table 2)
+	} >tmp &&
+	mv tmp "$idx" &&
+	clear_base &&
+	test_must_fail git cat-file blob $object &&
+	test_must_fail git index-pack --verify $pack
+'
+
+test_expect_success 'bogus OFS_DELTA in packfile' '
+	# Generate a pack with a delta in it.
+	base=$(test-genrandom foo 3000 | git hash-object --stdin -w) &&
+	delta=$(test-genrandom foo 2000 | git hash-object --stdin -w) &&
+	do_pack "$base $delta" --delta-base-offset &&
+	rm -f .git/objects/??/* &&
+
+	# Double check that we have the delta we expect.
+	echo $base >expect &&
+	echo $delta | git cat-file --batch-check="%(deltabase)" >actual &&
+	test_cmp expect actual &&
+
+	# Now corrupt it. We assume the varint size for the delta is small
+	# enough to fit in the first byte (which it should be, since it
+	# is a pure deletion from the base), and that original ofs_delta
+	# takes 2 bytes (which it should, as it should be ~3000).
+	ofs=$(git show-index <$idx | grep $delta | cut -d" " -f1) &&
+	munge $pack $(($ofs + 1)) "\177\377" &&
+	test_must_fail git cat-file blob $delta >/dev/null
+'
+
+test_done