Code: ecryptfs_decode_from_filename(unsigned char *dst, size_t *dst_size,
			      const unsigned char *src, size_t src_size)
{
	u8 current_bit_offset = 0;
	size_t src_byte_offset = 0;
	size_t dst_byte_offset = 0;

	if (dst == NULL) {
		(*dst_size) = ecryptfs_max_decoded_size(src_size);
		goto out;
	}
	while (src_byte_offset < src_size) {
		unsigned char src_byte =
				filename_rev_map[(int)src[src_byte_offset]];

		switch (current_bit_offset) {
		case 0:
			dst[dst_byte_offset] = (src_byte << 2);
			current_bit_offset = 6;
			break;
		case 6:
			dst[dst_byte_offset++] |= (src_byte >> 4);
			dst[dst_byte_offset] = ((src_byte & 0xF)
						 << 4);
			current_bit_offset = 4;
			break;
		case 4:
			dst[dst_byte_offset++] |= (src_byte >> 2);
			dst[dst_byte_offset] = (src_byte << 6);
			current_bit_offset = 2;
 			break;
 		case 2:
 			dst[dst_byte_offset++] |= (src_byte);
			dst[dst_byte_offset] = 0;
 			current_bit_offset = 0;
 			break;
 		}
		src_byte_offset++;
	}
	(*dst_size) = dst_byte_offset;
out:
	return;
}

Vulnerability Type: DoS Overflow +Priv 
CWE ID: CWE-189
Summary: Off-by-one error in the ecryptfs_decode_from_filename function in fs/ecryptfs/crypto.c in the eCryptfs subsystem in the Linux kernel before 3.18.2 allows local users to cause a denial of service (buffer overflow and system crash) or possibly gain privileges via a crafted filename.
Commit Message: eCryptfs: Remove buggy and unnecessary write in file name decode routine

Dmitry Chernenkov used KASAN to discover that eCryptfs writes past the
end of the allocated buffer during encrypted filename decoding. This
fix corrects the issue by getting rid of the unnecessary 0 write when
the current bit offset is 2.

Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Reported-by: Dmitry Chernenkov <dmitryc@google.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org # v2.6.29+: 51ca58d eCryptfs: Filename Encryption: Encoding and encryption functions
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>