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

On 2016-12-12 15:18, Paul Moore wrote:

On Mon, Dec 12, 2016 at 5:03 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);
        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)) {

I dislike the way you wrote this because instead of simply looking at
this to see if it correct I need to sort out all the bits and find out
if there are other error codes that could run afoul of this check ...
make it simple, e.g. (rc == -ENOMEM || rc == -EPERM || ...).
Actually, since EPERM is 1, -EPERM (-1 in two's compliment is
0xffffffff) is going to cause this to be true for pretty much any
value of rc, yes?

Yes, you are correct.  We need there a logical or on the results of each
comparison to the return code rather than bit-wise or-ing the result
codes together first to save a step.

quoted

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

The code in audit#next handles netlink_unicast() errors in
kauditd_thread() and you are adding error handling code here in
kauditd_send_unicast_skb() ... that's messy.  I don't care too much
where the auditd_reset() call is made, but let's only do it in one
function; FWIW, I originally put the error handling code in
kauditd_thread() because there was other error handling code that
needed to done in that scope so it resulted in cleaner code.

Hmmm, I seem to remember it not returning the return code and I thought
I had changed it to do so, but I see now that it was already there.
Agreed, I needlessly duplicated that error handling.

Related, I see you are now considering ENOMEM to be a fatal condition,
that differs from the AUDITD_BAD macro in kauditd_thread(); this
difference needs to be reconciled.

Also correct about -EPERM now that I check back to the intent of commit
32a1dbaece7e ("audit: try harder to send to auditd upon netlink
failure")

Finally, you should update the comment header block for auditd_reset()
that it needs to be called with the audit_cmd_mutex held.

Yup.

quoted

@@ -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))) {

Do we simply want to treat any error here as fatal, and not just
ECONN/EPERM/ENOMEM?  If not, let's come up with a single macro to
handle the fatal netlink_unicast() return codes so we have some chance
to keep things consistent in the future.

I'll work through this before I post another patch...

paul moore

- RGB