CVE-2018-8098: Integer Overflow or Wraparound (CWE-190)
value: Integer overflow in the index.c:read_entry() function while decompressing a compressed prefix length in libgit2 before v0.26.2 allows an attacker to cause a denial of service (out-of-bounds read) via a crafted repository index file.
CWE description: The product performs a calculation that can
         produce an integer overflow or wraparound when the logic
         assumes that the resulting value will always be larger than
         the original value. This occurs when an integer value is
         incremented to a value that is too large to store in the
         associated representation. When this occurs, the value may
         become a very small or negative number.
CVSS v3 base score: 6.5
Published: 2018-03-14T00:29Z
Repository: https://github.com/libgit2/libgit2
Language: C
Files changed: src/index.c
Commit date: 2018-03-10 10:23:26 +0000
Commit message: index: fix out-of-bounds read with invalid index entry prefix length

The index format in version 4 has prefix-compressed entries, where every
index entry can compress its path by using a path prefix of the previous
entry. Since implmenting support for this index format version in commit
5625d86b9 (index: support index v4, 2016-05-17), though, we do not
correctly verify that the prefix length that we want to reuse is
actually smaller or equal to the amount of characters than the length of
the previous index entry's path. This can lead to a an integer underflow
and subsequently to an out-of-bounds read.

Fix this by verifying that the prefix is actually smaller than the
previous entry's path length.

Reported-by: Krishna Ram Prakash R <krp@gtux.in>
Reported-by: Vivek Parikh <viv0411.parikh@gmail.com>

Diff: diff --git a/src/index.c b/src/index.c
index 3ef892b7f..20586f52c 100644
--- a/src/index.c
+++ b/src/index.c
@@ -2363,21 +2363,22 @@ static int read_entry(
 		}
 
 		entry_size = index_entry_size(path_length, 0, entry.flags);
 		entry.path = (char *)path_ptr;
 	} else {
-		size_t varint_len;
-		size_t strip_len = git_decode_varint((const unsigned char *)path_ptr,
-						     &varint_len);
-		size_t last_len = strlen(last);
-		size_t prefix_len = last_len - strip_len;
-		size_t suffix_len = strlen(path_ptr + varint_len);
-		size_t path_len;
-
-		if (varint_len == 0)
+		size_t varint_len, last_len, prefix_len, suffix_len, path_len;
+		uintmax_t strip_len;
+
+		strip_len = git_decode_varint((const unsigned char *)path_ptr, &varint_len);
+		last_len = strlen(last);
+
+		if (varint_len == 0 || last_len < strip_len)
 			return index_error_invalid("incorrect prefix length");
 
+		prefix_len = last_len - strip_len;
+		suffix_len = strlen(path_ptr + varint_len);
+
 		GITERR_CHECK_ALLOC_ADD(&path_len, prefix_len, suffix_len);
 		GITERR_CHECK_ALLOC_ADD(&path_len, path_len, 1);
 		tmp_path = git__malloc(path_len);
 		GITERR_CHECK_ALLOC(tmp_path);
 

Vulnerable code: 		size_t varint_len;
		size_t strip_len = git_decode_varint((const unsigned char *)path_ptr,
						     &varint_len);
		size_t last_len = strlen(last);
		size_t prefix_len = last_len - strip_len;
		size_t suffix_len = strlen(path_ptr + varint_len);
		size_t path_len;

		if (varint_len == 0)
Fixed code: 		size_t varint_len, last_len, prefix_len, suffix_len, path_len;
		uintmax_t strip_len;

		strip_len = git_decode_varint((const unsigned char *)path_ptr, &varint_len);
		last_len = strlen(last);

		if (varint_len == 0 || last_len < strip_len)
		prefix_len = last_len - strip_len;
		suffix_len = strlen(path_ptr + varint_len);