@matheusbsbs / matheus-bagliano / commits / 47fe3f6ef0

nth_packed_object_offset: bounds-check extended offset

If a pack .idx file has a corrupted offset for an object, we may try to access an offset in the .idx or .pack file that is larger than the file's size. For the .pack case, we have use_pack() to protect us, which realizes the access is out of bounds. But if the corrupted value asks us to look in the .idx file's secondary 64-bit offset table, we blindly add it to the mmap'd index data and access arbitrary memory. We can fix this with a simple bounds-check compared to the size we found when we opened the .idx file. Note that there's similar code in index-pack that is triggered only during "index-pack --verify". To support both, we pull the bounds-check into a separate function, which dies when it sees a corrupted file. It would be nice if we could return an error, so that the pack code could try to find a good copy of the object elsewhere. Currently nth_packed_object_offset doesn't have any way to return an error, but it could probably use "0" as a sentinel value (since no object can start there). This is the minimal fix, and we can improve the resilience later on top. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Jeff King committed Feb 25, 2016 at 09:22 UTC 47fe3f6ef0f5a336db90d816c5fb4330ffa23668
4 files changed +27 -1
builtin/index-pack.c
+1
@@ -1445,6 +1445,7 @@ static void read_v2_anomalous_offsets(struct packed_git *p,
1445 if (!(off & 0x80000000))
1446 continue;
1447 off = off & 0x7fffffff;
1448 + check_pack_index_ptr(p, &idx2[off * 2]);
1449 if (idx2[off * 2])
1450 continue;
1451 /*
cache.h
+10
@@ -1236,6 +1236,16 @@ extern void free_pack_by_name(const char *);
1236 extern void clear_delta_base_cache(void);
1237 extern struct packed_git *add_packed_git(const char *, int, int);
1238
1239 +/*
1240 + * Make sure that a pointer access into an mmap'd index file is within bounds,
1241 + * and can provide at least 8 bytes of data.
1242 + *
1243 + * Note that this is only necessary for variable-length segments of the file
1244 + * (like the 64-bit extended offset table), as we compare the size to the
1245 + * fixed-length parts when we open the file.
1246 + */
1247 +extern void check_pack_index_ptr(const struct packed_git *p, const void *ptr);
1248 +
1249 /*
1250 * Return the SHA-1 of the nth object within the specified packfile.
1251 * Open the index if it is not already open. The return value points
sha1_file.c
+15
@@ -2359,6 +2359,20 @@ const unsigned char *nth_packed_object_sha1(struct packed_git *p,
2359 }
2360 }
2361
2362 +void check_pack_index_ptr(const struct packed_git *p, const void *vptr)
2363 +{
2364 + const unsigned char *ptr = vptr;
2365 + const unsigned char *start = p->index_data;
2366 + const unsigned char *end = start + p->index_size;
2367 + if (ptr < start)
2368 + die("offset before start of pack index for %s (corrupt index?)",
2369 + p->pack_name);
2370 + /* No need to check for underflow; .idx files must be at least 8 bytes */
2371 + if (ptr >= end - 8)
2372 + die("offset beyond end of pack index for %s (truncated index?)",
2373 + p->pack_name);
2374 +}
2375 +
2376 off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
2377 {
2378 const unsigned char *index = p->index_data;
@@ -2372,6 +2386,7 @@ off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
2386 if (!(off & 0x80000000))
2387 return off;
2388 index += p->num_objects * 4 + (off & 0x7fffffff) * 8;
2389 + check_pack_index_ptr(p, index);
2390 return (((uint64_t)ntohl(*((uint32_t *)(index + 0)))) << 32) |
2391 ntohl(*((uint32_t *)(index + 4)));
2392 }
t/t5313-pack-bounds-checks.sh
+1 -1
@@ -128,7 +128,7 @@ test_expect_success 'bogus object offset (v2, no msb)' '
128 test_must_fail git index-pack --verify $pack
129 '
130
131 -test_expect_failure 'bogus offset into v2 extended table' '
131 +test_expect_success 'bogus offset into v2 extended table' '
132 do_pack $object --index-version=2 &&
133 munge $idx $(ofs_table 1) "\377\0\0\0" &&
134 clear_base &&