Re: [PATCH v2] audit: use proper refcount locking on audit_sock

On 2016-12-12 15:58, Cong Wang wrote:

On Mon, Dec 12, 2016 at 2:03 AM, Richard Guy Briggs [off-list ref] wrote:

quoted

Resetting audit_sock appears to be racy.

audit_sock was being copied and dereferenced without using a refcount on
the source sock.

Bump the refcount on the underlying sock when we store a refrence in
audit_sock and release it when we reset audit_sock.  audit_sock
modification needs the audit_cmd_mutex.

See: https://lkml.org/lkml/2016/11/26/232

Thanks to Eric Dumazet [off-list ref] and Cong Wang
[off-list ref] on ideas how to fix it.

Signed-off-by: Richard Guy Briggs <redacted>
---
There has been a lot of change in the audit code that is about to go
upstream to address audit queue issues.  This patch is based on the
source tree: git://git.infradead.org/users/pcmoore/audit#next
---
 kernel/audit.c |   34 ++++++++++++++++++++++++++++------
 1 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/kernel/audit.c b/kernel/audit.c
index f20eee0..439f7f3 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c

@@ -452,7 +452,9 @@ static void auditd_reset(void)
        struct sk_buff *skb;

        /* break the connection */
+       sock_put(audit_sock);

Why audit_sock can't be NULL here?

Fixed.

quoted

        audit_pid = 0;
+       audit_nlk_portid = 0;
        audit_sock = NULL;

        /* flush all of the retry queue to the hold queue */

@@ -478,6 +480,12 @@ static int kauditd_send_unicast_skb(struct sk_buff *skb)
        if (rc >= 0) {
                consume_skb(skb);
                rc = 0;
+       } else {
+               if (rc & (-ENOMEM|-EPERM|-ECONNREFUSED)) {

Are these errno's bits??

No, I've fixed this silly error.

quoted

+                       mutex_lock(&audit_cmd_mutex);
+                       auditd_reset();
+                       mutex_unlock(&audit_cmd_mutex);
+               }
        }

        return rc;

@@ -579,7 +587,9 @@ static int kauditd_thread(void *dummy)

                                auditd = 0;
                                if (AUDITD_BAD(rc, reschedule)) {
+                                       mutex_lock(&audit_cmd_mutex);
                                        auditd_reset();
+                                       mutex_unlock(&audit_cmd_mutex);
                                        reschedule = 0;
                                }
                        } else

@@ -594,7 +604,9 @@ static int kauditd_thread(void *dummy)
                                auditd = 0;
                                if (AUDITD_BAD(rc, reschedule)) {
                                        kauditd_hold_skb(skb);
+                                       mutex_lock(&audit_cmd_mutex);
                                        auditd_reset();
+                                       mutex_unlock(&audit_cmd_mutex);
                                        reschedule = 0;
                                } else
                                        /* temporary problem (we hope), queue

@@ -623,7 +635,9 @@ quick_loop:
                                if (rc) {
                                        auditd = 0;
                                        if (AUDITD_BAD(rc, reschedule)) {
+                                               mutex_lock(&audit_cmd_mutex);
                                                auditd_reset();
+                                               mutex_unlock(&audit_cmd_mutex);
                                                reschedule = 0;
                                        }

@@ -1004,17 +1018,22 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
                                return -EACCES;
                        }
                        if (audit_pid && new_pid &&
-                           audit_replace(requesting_pid) != -ECONNREFUSED) {
+                           (audit_replace(requesting_pid) & (-ECONNREFUSED|-EPERM|-ENOMEM))) {
                                audit_log_config_change("audit_pid", new_pid, audit_pid, 0);
                                return -EEXIST;
                        }
                        if (audit_enabled != AUDIT_OFF)
                                audit_log_config_change("audit_pid", new_pid, audit_pid, 1);
-                       audit_pid = new_pid;
-                       audit_nlk_portid = NETLINK_CB(skb).portid;
-                       audit_sock = skb->sk;
-                       if (!new_pid)
+                       if (new_pid) {
+                               if (audit_sock)
+                                       sock_put(audit_sock);
+                               audit_pid = new_pid;
+                               audit_nlk_portid = NETLINK_CB(skb).portid;
+                               sock_hold(skb->sk);

Why refcnt is still needed here? I need it because I removed the code
in net exit code path.

Because there is a chance that auditd exits abnormally and no message is
send from the kauditd thread to discover it has gone.

quoted

+                               audit_sock = skb->sk;
+                       } else {
                                auditd_reset();
+                       }
                        wake_up_interruptible(&kauditd_wait);
                }
                if (s.mask & AUDIT_STATUS_RATE_LIMIT) {

@@ -1283,8 +1302,11 @@ static void __net_exit audit_net_exit(struct net *net)
 {
        struct audit_net *aunet = net_generic(net, audit_net_id);
        struct sock *sock = aunet->nlsk;
-       if (sock == audit_sock)
+       if (sock == audit_sock) {
+               mutex_lock(&audit_cmd_mutex);

You need to put the if check inside the mutex too. Again, this could be
removed if you use refcnt.

Ok, right, fixed.

That last patch was a bit of a mess!  Thanks for your patience in
checking it...

quoted

                auditd_reset();
+               mutex_unlock(&audit_cmd_mutex);
+       }

        RCU_INIT_POINTER(aunet->nlsk, NULL);
        synchronize_net();
--
1.7.1

- RGB

--
Richard Guy Briggs [off-list ref]
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635