• Oleg Nesterov's avatar
    epoll: ep_unregister_pollwait() can use the freed pwq->whead · 971316f0
    Oleg Nesterov authored
    signalfd_cleanup() ensures that ->signalfd_wqh is not used, but
    this is not enough. eppoll_entry->whead still points to the memory
    we are going to free, ep_unregister_pollwait()->remove_wait_queue()
    is obviously unsafe.
    
    Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL,
    change ep_unregister_pollwait() to check pwq->whead != NULL under
    rcu_read_lock() before remove_wait_queue(). We add the new helper,
    ep_remove_wait_queue(), for this.
    
    This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because
    ->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand.
    ep_unregister_pollwait()->remove_wait_queue() can play with already
    freed and potentially reused ->sighand, but this is fine. This memory
    must have the valid ->signalfd_wqh until rcu_read_unlock().
    Reported-by: default avatarMaxime Bizon <mbizon@freebox.fr>
    Cc: <stable@kernel.org>
    Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    971316f0
eventpoll.c 50.1 KB