Re: netlink: GPF in sock_sndtimeo

On Fri, Dec 9, 2016 at 3:01 AM, Richard Guy Briggs [off-list ref] wrote:

On 2016-12-08 22:57, Cong Wang wrote:

quoted

On Thu, Dec 8, 2016 at 10:02 PM, Richard Guy Briggs [off-list ref] wrote:

quoted

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.

I will send a formal patch.

Thanks.