[Bug 234846] [lagg] race condition when adding port

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234846

            Bug ID: 234846
           Summary: [lagg] race condition when adding port
           Product: Base System
           Version: 11.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: alexandre.martins at stormshield.eu

Greetings,

I'm facing some random crash when I'm adding a port into an LACP aggregate.

In the coredump, I found that the crash is due to a race condition when the
interface is added to the aggregate.

The condition is the following:

The thread 1 (ifconfig lagg0 laggport em0) is adding an interface to an LACP
aggregate. The code enter the function lagg_port_create in the file if_lagg.c.
That function "publish" an update of the interface with the following code:

/* Change the interface type */
lp->lp_iftype = ifp->if_type;
ifp->if_type = IFT_IEEE8023ADLAG;
ifp->if_lagg = lp;
lp->lp_ioctl = ifp->if_ioctl;
ifp->if_ioctl = lagg_port_ioctl;
lp->lp_output = ifp->if_output;
ifp->if_output = lagg_port_output;

In the thread 2, thereafter that publication, a packet come on the interface
em0, use the new state of the interface, so lagg_input_p is called. Because the
setup of the LACP is not finished (the call to lagg_proto_addport is after the
publication), the LACP code uses a NULL pointer and the kernel crashes.

Can I change the code of the lagg_port_create function and put the
"publication" of the new state of the interface at the end of the function ?

Best regards,

Alexandre

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