Re: What's cooking in git.git (Jan 2019, #01; Mon, 7)

From: "Martin Ågren" <martin.agren@gmail.com>
To: Git Mailing List <git@vger.kernel.org>
Cc: "brian m. carlson" <sandals@crustytoothpaste.net>,
	Junio C Hamano <gitster@pobox.com>
Subject: Re: What's cooking in git.git (Jan 2019, #01; Mon, 7)
Date: Wed, 9 Jan 2019 22:06:08 +0100	[thread overview]
Message-ID: <CAN0heSqLUWpwRdeUvYj2KnDX-QxSOnWOdKWz77RjHKJ3AFUGEQ@mail.gmail.com> (raw)
In-Reply-To: <CAN0heSoRYYS3-UAamE9nibhORPoD+_TRHu5-ZTeYxYMS4BAnrA@mail.gmail.com>

On Wed, 9 Jan 2019 at 08:37, Martin Ågren <martin.agren@gmail.com> wrote:
>
> On Tue, 8 Jan 2019 at 00:34, Junio C Hamano <gitster@pobox.com> wrote:
> > * bc/sha-256 (2018-11-14) 12 commits
> >  - hash: add an SHA-256 implementation using OpenSSL
> >  - sha256: add an SHA-256 implementation using libgcrypt
> >  - Add a base implementation of SHA-256 support
> >  - commit-graph: convert to using the_hash_algo
> >  - t/helper: add a test helper to compute hash speed
> >  - sha1-file: add a constant for hash block size
> >  - t: make the sha1 test-tool helper generic
> >  - t: add basic tests for our SHA-1 implementation
> >  - cache: make hashcmp and hasheq work with larger hashes
> >  - hex: introduce functions to print arbitrary hashes
> >  - sha1-file: provide functions to look up hash algorithms
> >  - sha1-file: rename algorithm to "sha1"
> >
> >  Add sha-256 hash and plug it through the code to allow building Git
> >  with the "NewHash".
>
> AddressSanitizer barks at current pu (855f98be272f19d16564e) for a
> handful of tests.
>
> One example is t5702-protocol-v2.sh. [...]
>
> ==1691823==ERROR: AddressSanitizer: heap-buffer-overflow on address
> 0x6040000004f2 at pc 0x0000004ea0fd bp 0x7ffc53082590 sp
> 0x7ffc53081d40
> READ of size 32 at 0x6040000004f2 thread T0
>     #0 0x4ea0fc in __asan_memcpy
> llvm/projects/compiler-rt/lib/asan/asan_interceptors_memintrinsics.cc:23
>     #1 0x8603ec in oidset_insert oidset.c
>     #2 0x86c977 in add_promisor_object packfile.c:2129:4
>     #3 0x86c07a in for_each_object_in_pack packfile.c:2070:7
>     #4 0x86c535 in for_each_packed_object packfile.c:2095:7
>     #5 0x86c651 in is_promisor_object packfile.c:2151:4

> 0x6040000004f2 is located 0 bytes to the right of 34-byte region
> [0x6040000004d0,0x6040000004f2)
> allocated by thread T0 here:
>     #0 0x4eb4cf in malloc
> llvm/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:146
>     #1 0x9fa1db in do_xmalloc wrapper.c:60:8
>     #2 0x9fa2fd in do_xmallocz wrapper.c:100:8
>     #3 0x9fa2fd in xmallocz_gently wrapper.c:113
>     #4 0x86a877 in unpack_compressed_entry packfile.c:1588:11
>     #5 0x86a02e in unpack_entry packfile.c:1737:11
>     #6 0x867431 in cache_or_unpack_entry packfile.c:1439:10
>     #7 0x867431 in packed_object_info packfile.c:1506
>     #8 0x96b7be in oid_object_info_extended sha1-file.c:1394:10
>     #9 0x96d7d0 in read_object sha1-file.c:1434:6
>     #10 0x96d7d0 in read_object_file_extended sha1-file.c:1476
>     #11 0x85cf40 in repo_read_object_file ./object-store.h:174:9
>     #12 0x85cf40 in parse_object object.c:273
>     #13 0x86c752 in add_promisor_object packfile.c:2108:23
>     #14 0x86c07a in for_each_object_in_pack packfile.c:2070:7
>     #15 0x86c535 in for_each_packed_object packfile.c:2095:7
>     #16 0x86c651 in is_promisor_object packfile.c:2151:4

I found some more time to look into this.

It seems we have a buffer with raw data and we set up a `struct
object_id *` pointing into it, at a (supposed) OID value. Then
`update_tree_entry_internal()` verifies that the buffer contains
sufficiently many bytes, i.e., at least `the_hash_algo->rawsz` (=20).
We immediately call `oidset_insert()` which copies an entire struct,
i.e., we copy sizeof(struct object_id) (=32) bytes. Which is 12 more
than what is known to be safe. For this particular input data, we read
outside allocated memory.

I can think of three possible approaches:

* Allocate with a margin (GIT_MAX_RAWSZ - the_hash_algo->rawsz) where
  "necessary" (TM). Maybe not so maintainable.

* Teach `oidset_insert()` (i.e., khash) to only copy
  `the_hash_algo->rawsz` bytes. Maybe not so good for performance.

* Ignore.

I wonder which of these is the least awful, or if there are other ideas.

Martin