Commit message: delta: fix overflow when computing limit

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>
CVE: CVE-2018-10887
CWE: CWE-125
Vulnerability description: A flaw was found in libgit2 before version 0.27.3. It has been discovered that an unexpected sign extension in git_delta_apply function in delta.c file may lead to an integer overflow which in turn leads to an out of bound read, allowing to read before the base object. An attacker may use this flaw to leak memory addresses or cause a Denial of Service.

Project: libgit2
File: delta.c

int git_delta_apply(
	void **out,
	size_t *out_len,
	const unsigned char *base,
	size_t base_len,
	const unsigned char *delta,
	size_t delta_len)
{
	const unsigned char *delta_end = delta + delta_len;
	size_t base_sz, res_sz, alloc_sz;
	unsigned char *res_dp;

	*out = NULL;
	*out_len = 0;

	/*
	 * Check that the base size matches the data we were given;
	 * if not we would underflow while accessing data from the
	 * base object, resulting in data corruption or segfault.
	 */
	if ((hdr_sz(&base_sz, &delta, delta_end) < 0) || (base_sz != base_len)) {
		giterr_set(GITERR_INVALID, "failed to apply delta: base size does not match given data");
		return -1;
	}

	if (hdr_sz(&res_sz, &delta, delta_end) < 0) {
		giterr_set(GITERR_INVALID, "failed to apply delta: base size does not match given data");
		return -1;
	}

	GITERR_CHECK_ALLOC_ADD(&alloc_sz, res_sz, 1);
	res_dp = git__malloc(alloc_sz);
	GITERR_CHECK_ALLOC(res_dp);

	res_dp[res_sz] = '\0';
	*out = res_dp;
	*out_len = res_sz;

	while (delta < delta_end) {
 		unsigned char cmd = *delta++;
 		if (cmd & 0x80) {
 			/* cmd is a copy instruction; copy from the base. */
			size_t off = 0, len = 0;
 
 #define ADD_DELTA(o, shift) { if (delta < delta_end) (o) |= ((unsigned) *delta++ << shift); else goto fail; }
 			if (cmd & 0x01) ADD_DELTA(off, 0UL);
			if (cmd & 0x02) ADD_DELTA(off, 8UL);
			if (cmd & 0x04) ADD_DELTA(off, 16UL);
			if (cmd & 0x08) ADD_DELTA(off, 24UL);

			if (cmd & 0x10) ADD_DELTA(len, 0UL);
			if (cmd & 0x20) ADD_DELTA(len, 8UL);
			if (cmd & 0x40) ADD_DELTA(len, 16UL);
 			if (!len)       len = 0x10000;
 #undef ADD_DELTA
 
			if (base_len < off + len || res_sz < len)
 				goto fail;
 			memcpy(res_dp, base + off, len);
 			res_dp += len;
 			res_sz -= len;

		} else if (cmd) {
			/*
			 * cmd is a literal insert instruction; copy from
			 * the delta stream itself.
			 */
			if (delta_end - delta < cmd || res_sz < cmd)
				goto fail;
			memcpy(res_dp, delta, cmd);
			delta += cmd;
			res_dp += cmd;
			res_sz -= cmd;

		} else {
			/* cmd == 0 is reserved for future encodings. */
			goto fail;
		}
	}

	if (delta != delta_end || res_sz)
		goto fail;
	return 0;

fail:
	git__free(*out);

	*out = NULL;
	*out_len = 0;

	giterr_set(GITERR_INVALID, "failed to apply delta");
	return -1;
}