There are two different buffer overflows present when parsing "ok"
packets. First, we never verify whether the line already ends after
"ok", but directly go ahead and also try to skip the expected space
after "ok". Second, we then go ahead and use `strchr` to scan for the
terminating newline character. But in case where the line isn't
terminated correctly, this can overflow the line buffer.

Fix the issues by using `git__prefixncmp` to check for the "ok " prefix
and only checking for a trailing '\n' instead of using `memchr`. This
also fixes the issue of us always requiring a trailing '\n'.

Reported by oss-fuzz, issue 9749:

Crash Type: Heap-buffer-overflow READ {*}
Crash Address: 0x6310000389c0
Crash State:
  ok_pkt
  git_pkt_parse_line
  git_smart__store_refs

Sanitizer: address (ASAN)
(cherry picked from commit a9f1ca09)

We are being quite lenient when parsing "ACK" packets. First, we didn't
correctly verify that we're not overrunning the provided buffer length,
which we fix here by using `git__prefixncmp` instead of
`git__prefixcmp`. Second, we do not verify that the actual contents make
any sense at all, as we simply ignore errors when parsing the ACKs OID
and any unknown status strings. This may result in a parsed packet
structure with invalid contents, which is being silently passed to the
caller. This is being fixed by performing proper input validation and
checking of return codes.

(cherry picked from commit bc349045)

While the function parsing ref packets doesn't have any immediately
obvious buffer overflows, it's style is different to all the other
parsing functions. Instead of checking buffer length while we go, it
does a check up-front. This causes the code to seem a lot more magical
than it really is due to some magic constants. Refactor the function to
instead make use of the style of other packet parser and verify buffer
lengths as we go.

(cherry picked from commit 5edcf5d1)

In the `git_pkt_parse_line` function, we determine what kind of packet
a given packet line contains by simply checking for the prefix of that
line. Except for "ERR" packets, we always only check for the immediate
identifier without the trailing space (e.g. we check for an "ACK"
prefix, not for "ACK "). But for "ERR" packets, we do in fact include
the trailing space in our check. This is not really much of a problem at
all, but it is inconsistent with all the other packet types and thus
causes confusion when the `err_pkt` function just immediately skips the
space without checking whether it overflows the line buffer.

Adjust the check in `git_pkt_parse_line` to not include the trailing
space and instead move it into `err_pkt` for consistency.

(cherry picked from commit 786426ea)

When parsing data, progress or error packets, we need to copy the
contents of the rest of the current packet line into the flex-array of
the parsed packet. To keep track of this array's length, we then assign
the remaining length of the packet line to the structure. We do have a
mismatch of types here, as the structure's `len` field is a signed
integer, while the length that we are assigning has type `size_t`.

On nearly all platforms, this shouldn't pose any problems at all. The
line length can at most be 16^4, as the line's length is being encoded
by exactly four hex digits. But on a platforms with 16 bit integers,
this assignment could cause an overflow. While such platforms will
probably only exist in the embedded ecosystem, we still want to avoid
this potential overflow. Thus, we now simply change the structure's
`len` member to be of type `size_t` to avoid any integer promotion.

(cherry picked from commit 40fd84cc)

When we parse the packet type of an incoming packet line, we do not
verify that we don't overflow the provided line buffer. Fix this by
using `git__prefixncmp` instead and passing in `len`. As we have
previously already verified that `len <= linelen`, we thus won't ever
overflow the provided buffer length.

(cherry picked from commit 4a5804c9)

The commits following this commit are about to introduce quite a lot of
refactoring and tightening of the smart packet parser. Unfortunately, we
do not yet have any tests despite our online tests that verify that our
parser does not regress upon changes. This is doubly unfortunate as our
online tests aren't executed by default.

Add new tests that exercise the smart parsing logic directly by
executing `git_pkt_parse_line`.

(cherry picked from commit 365d2720)

If the remote sends a too-short packet, we'll allow `len` to go
negative and eventually issue a malloc for <= 0 bytes on

```
pkt->head.name = git__malloc(alloclen);
```

(cherry picked from commit 437ee5a7)

(cherry picked from commit 08961c9d)

(cherry picked from commit 895a668e)

(cherry picked from commit 90cf8607)

(cherry picked from commit 50dd7fea)

When running repack while doing repo writes, `packfile_load__cb()` can see some temporary files in the directory that are bigger than the usual, and makes `memcmp` overflow on the `p->pack_name` string. ASAN detected this. This just uses `strncmp`, that should not have any performance impact and is safe for comparing strings of different sizes.

```
ERROR: AddressSanitizer: heap-buffer-overflow on address 0x61200001a3f3 at pc 0x7f4a9e1976ec bp 0x7ffc1f80e100 sp 0x7ffc1f80d8b0
READ of size 89 at 0x61200001a3f3 thread T0
SCARINESS: 26 (multi-byte-read-heap-buffer-overflow)
    #0 0x7f4a9e1976eb in __interceptor_memcmp.part.78 (/build/cfgr-admin#link-tree/libtools_build_sanitizers_asan-ubsan-py.so+0xcf6eb)
    #1 0x7f4a518c5431 in packfile_load__cb /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb_pack.c:213
    #2 0x7f4a518d9582 in git_path_direach /build/libgit2/0.27.0/src/libgit2-0.27.0/src/path.c:1134
    #3 0x7f4a518c58ad in pack_backend__refresh /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb_pack.c:347
    #4 0x7f4a518c1b12 in git_odb_refresh /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:1511
    #5 0x7f4a518bff5f in git_odb__freshen /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:752
    #6 0x7f4a518c17d4 in git_odb_stream_finalize_write /build/libgit2/0.27.0/src/libgit2-0.27.0/src/odb.c:1415
    #7 0x7f4a51b9d015 in Repository_write /build/pygit2/0.27.0/src/pygit2-0.27.0/src/repository.c:509
```

(cherry picked from commit d22cd1f4)

The current error handling for the multiline variable parser is a bit
fragile, as each error condition has its own code to clear memory.
Instead, unify error handling as far as possible to avoid this
repetitive code. While at it, make use of `GITERR_CHECK_ALLOC` to
correctly handle OOM situations and verify that the buffer we print into
does not run out of memory either.

(cherry picked from commit bc63e1ef)

(cherry picked from commit 38b85255)

(cherry picked from commit a03113e8)

Release v0.27.4

OSS-fuzz has reported a potential out-of-bounds read when processing a
"ng" smart packet:

==1==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6310000249c0 at pc 0x000000493a92 bp 0x7ffddc882cd0 sp 0x7ffddc882480
	READ of size 65529 at 0x6310000249c0 thread T0
	SCARINESS: 26 (multi-byte-read-heap-buffer-overflow)
	#0 0x493a91 in __interceptor_strchr.part.35 /src/llvm/projects/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc:673
	#1 0x813960 in ng_pkt libgit2/src/transports/smart_pkt.c:320:14
	#2 0x810f79 in git_pkt_parse_line libgit2/src/transports/smart_pkt.c:478:9
	#3 0x82c3c9 in git_smart__store_refs libgit2/src/transports/smart_protocol.c:47:12
	#4 0x6373a2 in git_smart__connect libgit2/src/transports/smart.c:251:15
	#5 0x57688f in git_remote_connect libgit2/src/remote.c:708:15
	#6 0x52e59b in LLVMFuzzerTestOneInput /src/download_refs_fuzzer.cc:145:9
	#7 0x52ef3f in ExecuteFilesOnyByOne(int, char**) /src/libfuzzer/afl/afl_driver.cpp:301:5
	#8 0x52f4ee in main /src/libfuzzer/afl/afl_driver.cpp:339:12
	#9 0x7f6c910db82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
	#10 0x41d518 in _start

When parsing an "ng" packet, we keep track of both the current position
as well as the remaining length of the packet itself. But instead of
taking care not to exceed the length, we pass the current pointer's
position to `strchr`, which will search for a certain character until
hitting NUL. It is thus possible to create a crafted packet which
doesn't contain a NUL byte to trigger an out-of-bounds read.

Fix the issue by instead using `memchr`, passing the remaining length as
restriction. Furthermore, verify that we actually have enough bytes left
to produce a match at all.

OSS-Fuzz-Issue: 9406

Travis has upgraded the default Xcode images from 8.3 to 9.4 on 31st
July 2018, including an upgrade to macOS 10.13. Unfortunately, this
breaks our CI builds on our maintenance branches. As we do not want to
include mayor changes to fix the integration right now, we force use of
the old Xcode 8.3 images.

Release v0.27.3

When checking whether a delta base offset and length fit into the base
we have in memory already, we can trigger an overflow which breaks the
check. This would subsequently result in us reading memory from out of
bounds of the base.

The issue is easily fixed by checking for overflow when adding `off` and
`len`, thus guaranteeting that we are never indexing beyond `base_len`.
This corresponds to the git patch 8960844a7 (check patch_delta bounds
more carefully, 2006-04-07), which adds these overflow checks.

Reported-by: Riccardo Schirone <rschiron@redhat.com>

When computing the offset and length of the delta base, we repeatedly
increment the `delta` pointer without checking whether we have advanced
past its end already, which can thus result in an out-of-bounds read.
Fix this by repeatedly checking whether we have reached the end. Add a
test which would cause Valgrind to produce an error.

Reported-by: Riccardo Schirone <rschiron@redhat.com>
Test-provided-by: Riccardo Schirone <rschiron@redhat.com>

Our delta code was originally adapted from JGit, which itself adapted it
from git itself. Due to this heritage, we inherited a bug from git.git
in how we compute the delta offset, which was fixed upstream in
48fb7deb5 (Fix big left-shifts of unsigned char, 2009-06-17). As
explained by Linus:

    Shifting 'unsigned char' or 'unsigned short' left can result in sign
    extension errors, since the C integer promotion rules means that the
    unsigned char/short will get implicitly promoted to a signed 'int' due to
    the shift (or due to other operations).

    This normally doesn't matter, but if you shift things up sufficiently, it
    will now set the sign bit in 'int', and a subsequent cast to a bigger type
    (eg 'long' or 'unsigned long') will now sign-extend the value despite the
    original expression being unsigned.

    One example of this would be something like

            unsigned long size;
            unsigned char c;

            size += c << 24;

    where despite all the variables being unsigned, 'c << 24' ends up being a
    signed entity, and will get sign-extended when then doing the addition in
    an 'unsigned long' type.

    Since git uses 'unsigned char' pointers extensively, we actually have this
    bug in a couple of places.

In our delta code, we inherited such a bogus shift when computing the
offset at which the delta base is to be found. Due to the sign extension
we can end up with an offset where all the bits are set. This can allow
an arbitrary memory read, as the addition in `base_len < off + len` can
now overflow if `off` has all its bits set.

Fix the issue by casting the result of `*delta++ << 24UL` to an unsigned
integer again. Add a test with a crafted delta that would actually
succeed with an out-of-bounds read in case where the cast wouldn't
exist.

Reported-by: Riccardo Schirone <rschiron@redhat.com>
Test-provided-by: Riccardo Schirone <rschiron@redhat.com>

Bugfix release v0.27.2

The test submodule::lookup::duplicated_path, which tries to verify that
we detect submodules with duplicated paths, currently relies on the
gitmodules file of "submod2_target". While this file has two gitmodules
with the same path, one of these gitmodules has an empty name and thus
does not pass `git_submodule_name_is_valid`. Because of this, the test
is in fact dependent on the iteration order in which we process the
submodules. In fact the "valid" submodule comes first, the "invalid"
submodule will cause the desired error. In fact the "invalid" submodule
comes first, it will be skipped due to its name being invalid, and we
will not see the desired error. While this works on the master branch
just right due to the refactoring of our config code, where iteration
order is now deterministic, this breaks on all older maintenance
branches.

Fix the issue by simply using `cl_git_rewritefile` to rewrite the
gitmodules file. This greatly simplifies the test and also makes the
intentions of it much clearer.

When loading submodule names, we build a map of submodule paths and
their respective names. While looping over the configuration keys,
we do not check though whether a submodule path was seen already. This
leads to a memory leak in case we have multiple submodules with the same
path, as we just overwrite the old value in the map in that case.

Fix the error by verifying that the path to be added is not yet part of
the string map. Git does not allow to have multiple submodules for a
path anyway, so we now do the same and detect this duplication,
reporting it to the user.

With the recent change of always resolving pkg-config libraries to their
full path, we do not have to manage the LIBGIT2_LIBDIRS variable
anymore. The only other remaining user of LIBGIT2_LIBDIRS is winhttp,
which is a CMake-style library target and can thus be resolved by CMake
automatically.

Remove the variable to simplify our build system a bit.

Libraries found by CMake modules are usually handled with their full
path. This makes linking against those libraries a lot more robust when
it comes to libraries in non-standard locations, as otherwise we might
mix up libraries from different locations when link directories are
given.

One excemption are libraries found by PKG_CHECK_MODULES. Instead of
returning libraries with their complete path, it will return the
variable names as well as a set of link directories. In case where
multiple sets of the same library are installed in different locations,
this can lead the compiler to link against the wrong libraries in the
end, when link directories of other dependencies are added.

To fix this shortcoming, we need to manually resolve library paths
returned by CMake against their respective library directories. This is
an easy task to do with `FIND_LIBRARY`.

Valgrind log :

==17702== 18 bytes in 1 blocks are indirectly lost in loss record 69 of 1,123
==17702==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==17702==    by 0x5FDBB49: strdup (strdup.c:42)
==17702==    by 0x632B3E: git__strdup (util.h:106)
==17702==    by 0x632D2C: git_reference__alloc_symbolic (refs.c:64)
==17702==    by 0x62E0AF: loose_lookup (refdb_fs.c:408)
==17702==    by 0x62E636: refdb_fs_backend__iterator_next (refdb_fs.c:565)
==17702==    by 0x62CD8E: git_refdb_iterator_next (refdb.c:147)
==17702==    by 0x6347F2: git_reference_next (refs.c:838)
==17702==    by 0x6345CB: git_reference_foreach (refs.c:748)
==17702==    by 0x66BE62: local_download_pack (local.c:579)
==17702==    by 0x5DB48F: git_fetch_download_pack (fetch.c:148)
==17702==    by 0x639028: git_remote_download (remote.c:932)
==17702==    by 0x63919A: git_remote_fetch (remote.c:969)
==17702==    by 0x4ABEDD: test_fetchhead_nonetwork__fetch_into_repo_with_symrefs (nonetwork.c:362)
==17702==    by 0x4125D9: clar_run_test (clar.c:222)
==17702==    by 0x41287C: clar_run_suite (clar.c:286)
==17702==    by 0x412DDE: clar_test_run (clar.c:433)
==17702==    by 0x4105E1: main (main.c:24)

As per CID:1378747, we might be called with a NULL repo, which would be deferenced in write_add_refspec

Signed-off-by: Sven Strickroth <email@cs-ware.de>

CID:1383993, "In git_refspec__dwim_one: All paths that lead to this null pointer comparison already dereference the pointer earlier (CWE-476)"

Commit 723e1e97 (appveyor: disable DHE to avoid spurious failures,
2018-03-29) added a workaround to fix spurious test failures due to a
bug in Windows' SChannel implementation. The workaround only worked by
accident, though, as the registry key was in fact mistyped. Fix the
typo.