iscsi target: Use hex2bin instead of a re-implementation

This change has the following effects, in order of descreasing importance:
1) Prevent a stack buffer overflow.
2) Do not append an unnecessary NULL to an anyway binary buffer, which
   is writing one byte past client_digest when caller is:
     chap_string_to_hex(client_digest, chap_r, strlen(chap_r));

The latter was found by KASAN (see below) when input value hes expected
size (32 hex chars), and further analysis revealed a stack buffer overflow
can  happen when network-received value is longer, allowing an
unauthenticated remote attacker to smash up to 17 bytes after destination
buffer (16 bytes attacker-controlled and one null).
As switching to hex2bin requires specifying destination buffer length, and
does not internally append any null, it solves both issues.

This addresses CVE-2018-14633.

Beyond this:
- Validate received value length and check hex2bin accepted the input, to
  log this rejection reason instead of just failing authentication.
- Only log received CHAP_R and CHAP_C values once they passed sanity
  checks.

==================================================================
BUG: KASAN: stack-out-of-bounds in chap_string_to_hex+0x32/0x60 [iscsi_target_mod]
Write of size 1 at addr ffff8801090ef7c8 by task kworker/0:0/1021

CPU: 0 PID: 1021 Comm: kworker/0:0 Tainted: G           O      4.17.8kasan.sess.connops+ #2
Hardware name: To be filled by O.E.M. To be filled by O.E.M./Aptio CRB, BIOS 5.6.5 05/19/2014
Workqueue: events iscsi_target_do_login_rx [iscsi_target_mod]
Call Trace:
 dump_stack+0x71/0xac
 print_address_description+0x65/0x22e
 ? chap_string_to_hex+0x32/0x60 [iscsi_target_mod]
 kasan_report.cold.6+0x241/0x2fd
 chap_string_to_hex+0x32/0x60 [iscsi_target_mod]
 chap_server_compute_md5.isra.2+0x2cb/0x860 [iscsi_target_mod]
 ? chap_binaryhex_to_asciihex.constprop.5+0x50/0x50 [iscsi_target_mod]
 ? ftrace_caller_op_ptr+0xe/0xe
 ? __orc_find+0x6f/0xc0
 ? unwind_next_frame+0x231/0x850
 ? kthread+0x1a0/0x1c0
 ? ret_from_fork+0x35/0x40
 ? ret_from_fork+0x35/0x40
 ? iscsi_target_do_login_rx+0x3bc/0x4c0 [iscsi_target_mod]
 ? deref_stack_reg+0xd0/0xd0
 ? iscsi_target_do_login_rx+0x3bc/0x4c0 [iscsi_target_mod]
 ? is_module_text_address+0xa/0x11
 ? kernel_text_address+0x4c/0x110
 ? __save_stack_trace+0x82/0x100
 ? ret_from_fork+0x35/0x40
 ? save_stack+0x8c/0xb0
 ? 0xffffffffc1660000
 ? iscsi_target_do_login+0x155/0x8d0 [iscsi_target_mod]
 ? iscsi_target_do_login_rx+0x3bc/0x4c0 [iscsi_target_mod]
 ? process_one_work+0x35c/0x640
 ? worker_thread+0x66/0x5d0
 ? kthread+0x1a0/0x1c0
 ? ret_from_fork+0x35/0x40
 ? iscsi_update_param_value+0x80/0x80 [iscsi_target_mod]
 ? iscsit_release_cmd+0x170/0x170 [iscsi_target_mod]
 chap_main_loop+0x172/0x570 [iscsi_target_mod]
 ? chap_server_compute_md5.isra.2+0x860/0x860 [iscsi_target_mod]
 ? rx_data+0xd6/0x120 [iscsi_target_mod]
 ? iscsit_print_session_params+0xd0/0xd0 [iscsi_target_mod]
 ? cyc2ns_read_begin.part.2+0x90/0x90
 ? _raw_spin_lock_irqsave+0x25/0x50
 ? memcmp+0x45/0x70
 iscsi_target_do_login+0x875/0x8d0 [iscsi_target_mod]
 ? iscsi_target_check_first_request.isra.5+0x1a0/0x1a0 [iscsi_target_mod]
 ? del_timer+0xe0/0xe0
 ? memset+0x1f/0x40
 ? flush_sigqueue+0x29/0xd0
 iscsi_target_do_login_rx+0x3bc/0x4c0 [iscsi_target_mod]
 ? iscsi_target_nego_release+0x80/0x80 [iscsi_target_mod]
 ? iscsi_target_restore_sock_callbacks+0x130/0x130 [iscsi_target_mod]
 process_one_work+0x35c/0x640
 worker_thread+0x66/0x5d0
 ? flush_rcu_work+0x40/0x40
 kthread+0x1a0/0x1c0
 ? kthread_bind+0x30/0x30
 ret_from_fork+0x35/0x40

The buggy address belongs to the page:
page:ffffea0004243bc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x17fffc000000000()
raw: 017fffc000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: ffffea0004243c20 ffffea0004243ba0 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801090ef680: f2 f2 f2 f2 f2 f2 f2 01 f2 f2 f2 f2 f2 f2 f2 00
 ffff8801090ef700: f2 f2 f2 f2 f2 f2 f2 00 02 f2 f2 f2 f2 f2 f2 00
>ffff8801090ef780: 00 f2 f2 f2 f2 f2 f2 00 00 f2 f2 f2 f2 f2 f2 00
                                              ^
 ffff8801090ef800: 00 f2 f2 f2 f2 f2 f2 00 00 00 00 02 f2 f2 f2 f2
 ffff8801090ef880: f2 f2 f2 00 00 00 00 00 00 00 00 f2 f2 f2 f2 00
==================================================================

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
CVE-2018-14633

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/drivers/target/iscsi/iscsi_target_auth.c b/drivers/target/iscsi/iscsi_target_auth.c
index 9518ffd8b8bac81cc1b725046857d5f229e58b15..6c3b4c02289404eacc49a16b42778ae08e1eb4f0 100644 (file)
--- a/drivers/target/iscsi/iscsi_target_auth.c
+++ b/drivers/target/iscsi/iscsi_target_auth.c
@@ -26,18 +26,6 @@
 #include "iscsi_target_nego.h"
 #include "iscsi_target_auth.h"
 
-static int chap_string_to_hex(unsigned char *dst, unsigned char *src, int len)
-{
-       int j = DIV_ROUND_UP(len, 2), rc;
-
-       rc = hex2bin(dst, src, j);
-       if (rc < 0)
-               pr_debug("CHAP string contains non hex digit symbols\n");
-
-       dst[j] = '\0';
-       return j;
-}
-
 static void chap_binaryhex_to_asciihex(char *dst, char *src, int src_len)
 {
        int i;
@@ -248,9 +236,16 @@ static int chap_server_compute_md5(
                pr_err("Could not find CHAP_R.\n");
                goto out;
        }
+       if (strlen(chap_r) != MD5_SIGNATURE_SIZE * 2) {
+               pr_err("Malformed CHAP_R\n");
+               goto out;
+       }
+       if (hex2bin(client_digest, chap_r, MD5_SIGNATURE_SIZE) < 0) {
+               pr_err("Malformed CHAP_R\n");
+               goto out;
+       }
 
        pr_debug("[server] Got CHAP_R=%s\n", chap_r);
-       chap_string_to_hex(client_digest, chap_r, strlen(chap_r));
 
        tfm = crypto_alloc_shash("md5", 0, 0);
        if (IS_ERR(tfm)) {
@@ -349,9 +344,7 @@ static int chap_server_compute_md5(
                pr_err("Could not find CHAP_C.\n");
                goto out;
        }
-       pr_debug("[server] Got CHAP_C=%s\n", challenge);
-       challenge_len = chap_string_to_hex(challenge_binhex, challenge,
-                               strlen(challenge));
+       challenge_len = DIV_ROUND_UP(strlen(challenge), 2);
        if (!challenge_len) {
                pr_err("Unable to convert incoming challenge\n");
                goto out;
@@ -360,6 +353,11 @@ static int chap_server_compute_md5(
                pr_err("CHAP_C exceeds maximum binary size of 1024 bytes\n");
                goto out;
        }
+       if (hex2bin(challenge_binhex, challenge, challenge_len) < 0) {
+               pr_err("Malformed CHAP_C\n");
+               goto out;
+       }
+       pr_debug("[server] Got CHAP_C=%s\n", challenge);
        /*
         * During mutual authentication, the CHAP_C generated by the
         * initiator must not match the original CHAP_C generated by