RISE-2009003
Linux eCryptfs parse_tag_3_packet Encrypted Key Buffer Overflow Vulnerability

http://risesecurity.org/advisories/RISE-2009003.txt
Published: July 28, 2009
Updated: July 28, 2009

INTRODUCTION

There exists a vulnerability within a function of Linux eCryptfs (Enterprise
Cryptographic Filesystem), which when properly exploited can lead to
compromise of the vulnerable system. This vulnerability was confirmed by us in
the Linux kernel version 2.6.30.3. Linux kernel versions 2.6.19 and later have
eCryptfs support and may be also affected.

DETAILS

eCryptfs is a POSIX-compliant enterprise-class stacked cryptographic
filesystem for Linux.

It is derived from Erez Zadok's Cryptfs, implemented through the FiST
framework for generating stacked filesystems. eCryptfs extends Cryptfs to
provide advanced key management and policy features. eCryptfs stores
cryptographic metadata in the header of each file written, so that encrypted
files can be copied between hosts; the file will be decryptable with the
proper key, and there is no need to keep track of any additional information
aside from what is already in the encrypted file itself. Think of eCryptfs as
a sort of ``gnupgfs.''

The parse_tag_3_packet function of eCryptfs in-kernel key management code does
not check if the tag 3 packet contains a encrypted key size larger than
ECRYPTFS_MAX_ENCRYPTED_KEY_BYTES, before copying the encrypted key to the
new_auth_tok structure, resulting in a kernel heap-based buffer overflow
vulnerability.

fs/ecryptfs/keystore.c
--
static int
parse_tag_3_packet(struct ecryptfs_crypt_stat *crypt_stat,
           unsigned char *data, struct list_head *auth_tok_list,
           struct ecryptfs_auth_tok **new_auth_tok,
           size_t *packet_size, size_t max_packet_size)
{
    size_t body_size;
    struct ecryptfs_auth_tok_list_item *auth_tok_list_item;
    size_t length_size;
    int rc = 0;

    ...

    /* Released: wipe_auth_tok_list called in ecryptfs_parse_packet_set or
     * at end of function upon failure */
    auth_tok_list_item =
        kmem_cache_zalloc(ecryptfs_auth_tok_list_item_cache, GFP_KERNEL);
    if (!auth_tok_list_item) {
        printk(KERN_ERR "Unable to allocate memory\n");
        rc = -ENOMEM;
        goto out;
    }
    (*new_auth_tok) = &auth_tok_list_item->auth_tok;
    rc = ecryptfs_parse_packet_length(&data[(*packet_size)], &body_size,
                      &length_size);
    if (rc) {
        printk(KERN_WARNING "Error parsing packet length; rc = [%d]\n",
               rc);
        goto out_free;
    }

    ...

    (*new_auth_tok)->session_key.encrypted_key_size =
        (body_size - (ECRYPTFS_SALT_SIZE + 5));
    if (unlikely(data[(*packet_size)++] != 0x04)) {
        printk(KERN_WARNING "Unknown version number [%d]\n",
               data[(*packet_size) - 1]);
        rc = -EINVAL;
        goto out_free;
    }

    ...

        /* Friendly reminder:
         * (*new_auth_tok)->session_key.encrypted_key_size =
         *         (body_size - (ECRYPTFS_SALT_SIZE + 5)); */
        memcpy((*new_auth_tok)->session_key.encrypted_key,
               &data[(*packet_size)],
               (*new_auth_tok)->session_key.encrypted_key_size);
        (*packet_size) +=
            (*new_auth_tok)->session_key.encrypted_key_size;

    ...
--

VENDOR

A patch for this vulnerability was sent to the Linux kernel mailing list by
Ramon de Carvalho Valle <ramon@risesecurity.org> and Tyler Hicks
<tyhicks@linux.vnet.ibm.com>.

CREDITS

This vulnerability was discovered by Ramon de Carvalho Valle
<ramon@risesecurity.org>.

DISCLAIMER

The authors reserve the right not to be responsible for the topicality,
correctness, completeness or quality of the information provided in this
document. Liability claims regarding damage caused by the use of any
information provided, including any kind of information which is incomplete or
incorrect, will therefore be rejected.