From 47fe3f6ef0f5a336db90d816c5fb4330ffa23668 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 25 Feb 2016 09:22:52 -0500 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- builtin/index-pack.c | 1 + cache.h | 10 ++++++++++ sha1_file.c | 15 +++++++++++++++ t/t5313-pack-bounds-checks.sh | 2 +- 4 files changed, 27 insertions(+), 1 deletion(-) diff --git a/builtin/index-pack.c b/builtin/index-pack.c index 723fe8e11d..98bdbb5e6c 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -1445,6 +1445,7 @@ 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; /* diff --git a/cache.h b/cache.h index 4427945bc0..6c9aaa1ae6 100644 --- a/cache.h +++ b/cache.h @@ -1237,6 +1237,16 @@ extern void clear_delta_base_cache(void); extern struct packed_git *add_packed_git(const char *, int, int); /* + * 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 * at the SHA-1 within the mmapped index. Return NULL if there is an diff --git a/sha1_file.c b/sha1_file.c index 99155c0d6b..bd0f8f7c8d 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -2359,6 +2359,20 @@ 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; @@ -2372,6 +2386,7 @@ 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))); } diff --git a/t/t5313-pack-bounds-checks.sh b/t/t5313-pack-bounds-checks.sh index efc5197345..0717746479 100755 --- a/t/t5313-pack-bounds-checks.sh +++ b/t/t5313-pack-bounds-checks.sh @@ -128,7 +128,7 @@ test_expect_success 'bogus object offset (v2, no msb)' ' test_must_fail git index-pack --verify $pack ' -test_expect_failure 'bogus offset into v2 extended table' ' +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 && -- 2.11.4.GIT