Description of problem:
There is a bug in snd_seq_oss_open from sound/core/seq/oss/seq_oss_init.c. So here's the error path for some setup failure:

 _error:
    snd_seq_oss_writeq_delete(dp->writeq);
    snd_seq_oss_readq_delete(dp->readq);
    snd_seq_oss_synth_cleanup(dp);
    snd_seq_oss_midi_cleanup(dp);
    delete_port(dp);
    delete_seq_queue(dp->queue);
    kfree(dp);

This looks okay, but actually, delete_port calls port_delete (eventually...
this code is tough to follow) which does a free_devinfo on the owner struct
seq_oss_devinfo, here (around ~269 in seq_ports.c):

    if (port->private_free)
        port->private_free(port->private_data);

because of this (around ~334 in seq_oss_init.c):

    memset(&callback, 0, sizeof(callback));
    callback.owner = THIS_MODULE;
    callback.private_data = dp;
    callback.event_input = snd_seq_oss_event_input;
    callback.private_free = free_devinfo;
    port.kernel = &callback;

Which does this:

static void
free_devinfo(void *private)
{
    struct seq_oss_devinfo *dp = (struct seq_oss_devinfo *)private;

    if (dp->timer)
        snd_seq_oss_timer_delete(dp->timer);
        
    if (dp->writeq)
        snd_seq_oss_writeq_delete(dp->writeq);

    if (dp->readq)
        snd_seq_oss_readq_delete(dp->readq);
    
    kfree(dp);
}

So.....

    delete_port(dp);
    delete_seq_queue(dp->queue); <= Oops, dereferencing released pointer.
    kfree(dp);                   <= Oops, double free.

Acknowledgements:

Red Hat would like to thank Tavis Ormandy for reporting this issue.