Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [day] [month] [year] [list]
Date: Mon, 24 Sep 2018 06:03:44 -0400 (EDT)
From: Vladis Dronov <vdronov@...hat.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2018-14633: Linux kernel: security flaw in iscsi target code

Heololo,

"Vincent Pelletier" <plr.vincent@...il.com> has found a security flaw in the
chap_server_compute_md5() function in the ISCSI target code in the Linux kernel
in a way an authentication request from an ISCSI initiator is processed. An
unauthenticated remote attacker can cause a stack buffer overflow and smash up
to 17 bytes of the stack. The attack requires the iSCSI target to be enabled on
the victim host.

This flaw was assigned a CVE-ID of CVE-2018-14633 and I would suggest to use it
in the future communications re: this:

https://bugzilla.redhat.com/show_bug.cgi?id=1626035

The suggested patches:

https://git.kernel.org/pub/scm/linux/kernel/git/mkp/scsi.git/commit/?h=4.19/scsi-fixes&id=1816494330a83f2a064499d8ed2797045641f92c

https://git.kernel.org/pub/scm/linux/kernel/git/mkp/scsi.git/commit/?h=4.19/scsi-fixes&id=8c39e2699f8acb2e29782a834e56306da24937fe

The impact analysis follows. The flaw resides in this code:

[drivers/target/iscsi/iscsi_target_auth.h]
#define CHAP_CHALLENGE_LENGTH  16
#define MD5_SIGNATURE_SIZE     16      /* 16 bytes in a MD5 message digest */
#define MAX_RESPONSE_LENGTH    64      /* sufficient for MD5 */
#define MAX_CHAP_N_SIZE        512

[drivers/target/iscsi/iscsi_target_auth.c]
static int chap_server_compute_md5( ... char *nr_in_ptr, char *nr_out_ptr, ... )
{       ...
        unsigned char client_digest[MD5_SIGNATURE_SIZE];
        unsigned char server_digest[MD5_SIGNATURE_SIZE];
        unsigned char chap_r[MAX_RESPONSE_LENGTH];
        ...
        if (extract_param(nr_in_ptr, "CHAP_R", MAX_RESPONSE_LENGTH, chap_r,
            &type) < 0) { ...exit... }
        ...
        chap_string_to_hex(client_digest, chap_r, strlen(chap_r));
        ...
        // int crypto_shash_finup(struct shash_desc *desc, const u8 *data,
        //     unsigned int len, u8 *out)
        // note, server_digest is *out
        ret = crypto_shash_finup(desc, chap->challenge, CHAP_CHALLENGE_LENGTH, server_digest);
        ...
        chap_binaryhex_to_asciihex(response, server_digest, MD5_SIGNATURE_SIZE);

Here chap_string_to_hex() (which basically does hex2bin()) can have max 64-bytes
input string, then it converts the input to a 32-bytes binary string and writes
it plus the trailing \0 to the 16-bytes on-stack buffer client_digest[], making
this a classical buffer overflow.

chap_r is attacker-controlled, the question is what an attack can overwrite.
The overwrite can be 16 bytes + zero byte. Checking what is where on the stack
in, for example, RHEL-7/x86_64, server_digest[] and the 1st byte of response[]
can be overwritten. According to the code, server_digest[] and response[] are
not used after the overwrite and are filled with correct values later by the
crypto_shash_finup() and the chap_binaryhex_to_asciihex().

This means the flaw has no impact on the RHEL-7/x86_64 systems.

Depending on how the other distriutions' kernel is built (i.e. depending on a
compiler, compiler flags and hardware architecture) a compiler may put other
local variables or function arguments on the stack after the client_digest. This
may lead to different outcomes, like chap_server_compute_md5() may erroneously
return with a result of a successful authentication by rewriting auth_ret, and
so expose all the target's content to an attacker. Or, if nr_out_ptr is
overwritten, this can damage other kernel memory content via later sprintf() and
thus lead to a system crash.

At last, with distribution kernels where stack canary is disabled the overwrite
may corrupt a return pointer, saved registers and other function's stack frame.
Due to this, privilege escalation cannot be fully ruled out, although we believe
this is highly unlikely.

Best regards,
Vladis Dronov | Red Hat, Inc. | Product Security Engineer

Powered by blists - more mailing lists

Your e-mail address:

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.