Re: netlink: GPF in sock_sndtimeo

[Cong Wang]

On Fri, Dec 9, 2016 at 8:13 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> On Fri, Dec 9, 2016 at 3:01 AM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>> On 2016-12-08 22:57, Cong Wang wrote:
>>> On Thu, Dec 8, 2016 at 10:02 PM, Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>>> > I also tried to extend Cong Wang's idea to attempt to proactively respond to a
>>> > NETLINK_URELEASE on the audit_sock and reset it, but ran into a locking error
>>> > stack dump using mutex_lock(&audit_cmd_mutex) in the notifier callback.
>>> > Eliminating the lock since the sock is dead anways eliminates the error.
>>> >
>>> > Is it safe?  I'll resubmit if this looks remotely sane.  Meanwhile I'll try to
>>> > get the test case to compile.
>>>
>>> It doesn't look safe, because 'audit_sock', 'audit_nlk_portid' and 'audit_pid'
>>> are updated as a whole and race between audit_receive_msg() and
>>> NETLINK_URELEASE.
>>
>> This is what I expected and why I originally added the mutex lock in the
>> callback...  The dumps I got were bare with no wrapper identifying the
>> process context or specific error, so I'm at a bit of a loss how to
>> solve this (without thinking more about it) other than instinctively
>> removing the mutex.
>
> Netlink notifier can safely be converted to blocking one, I will send
> a patch.
>
> But I seriously doubt you really need NETLINK_URELEASE here,
> it adds nothing but overhead, b/c the netlink notifier is called on
> every netlink socket in the system, but for net exit path, that is
> relatively a slow path.
>
> Also, kauditd_send_skb() needs audit_cmd_mutex too.

Please let me know what you think about the attached patch?

Thanks!
commit a12b43ee814625933ff155c20dc863c59cfcf240
Author: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Date:   Fri Dec 9 17:56:42 2016 -0800

    audit: close a race condition on audit_sock
    
    Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>

diff --git a/kernel/audit.c b/kernel/audit.c
index f1ca116..ab947d8 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -423,6 +423,8 @@ static void kauditd_send_skb(struct sk_buff *skb)
 				snprintf(s, sizeof(s), "audit_pid=%d reset", audit_pid);
 				audit_log_lost(s);
 				audit_pid = 0;
+				audit_nlk_portid = 0;
+				sock_put(audit_sock);
 				audit_sock = NULL;
 			} else {
 				pr_warn("re-scheduling(#%d) write to audit_pid=%d\n",
@@ -899,6 +901,9 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 				audit_log_config_change("audit_pid", new_pid, audit_pid, 1);
 			audit_pid = new_pid;
 			audit_nlk_portid = NETLINK_CB(skb).portid;
+			sock_hold(skb->sk);
+			if (audit_sock)
+				sock_put(audit_sock);
 			audit_sock = skb->sk;
 		}
 		if (s.mask & AUDIT_STATUS_RATE_LIMIT) {
@@ -1167,10 +1172,6 @@ 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) {
-		audit_pid = 0;
-		audit_sock = NULL;
-	}
 
 	RCU_INIT_POINTER(aunet->nlsk, NULL);
 	synchronize_net();