Doug / smarc-fsl-linux-kernel

11 Oct, 2007

1 commit

11 Sep, 2007

1 commit

  • 16fcec35e   [NETFILTER]: Fix/improve deadlock condition on module removal netfilter ... Browse Code »

    So I've had a deadlock reported to me. I've found that the sequence of
    events goes like this:

    1) process A (modprobe) runs to remove ip_tables.ko

    2) process B (iptables-restore) runs and calls setsockopt on a netfilter socket,
    increasing the ip_tables socket_ops use count

    3) process A acquires a file lock on the file ip_tables.ko, calls remove_module
    in the kernel, which in turn executes the ip_tables module cleanup routine,
    which calls nf_unregister_sockopt

    4) nf_unregister_sockopt, seeing that the use count is non-zero, puts the
    calling process into uninterruptible sleep, expecting the process using the
    socket option code to wake it up when it exits the kernel

    4) the user of the socket option code (process B) in do_ipt_get_ctl, calls
    ipt_find_table_lock, which in this case calls request_module to load
    ip_tables_nat.ko

    5) request_module forks a copy of modprobe (process C) to load the module and
    blocks until modprobe exits.

    6) Process C. forked by request_module process the dependencies of
    ip_tables_nat.ko, of which ip_tables.ko is one.

    7) Process C attempts to lock the request module and all its dependencies, it
    blocks when it attempts to lock ip_tables.ko (which was previously locked in
    step 3)

    Theres not really any great permanent solution to this that I can see, but I've
    developed a two part solution that corrects the problem

    Part 1) Modifies the nf_sockopt registration code so that, instead of using a
    use counter internal to the nf_sockopt_ops structure, we instead use a pointer
    to the registering modules owner to do module reference counting when nf_sockopt
    calls a modules set/get routine. This prevents the deadlock by preventing set 4
    from happening.

    Part 2) Enhances the modprobe utilty so that by default it preforms non-blocking
    remove operations (the same way rmmod does), and add an option to explicity
    request blocking operation. So if you select blocking operation in modprobe you
    can still cause the above deadlock, but only if you explicity try (and since
    root can do any old stupid thing it would like.... :) ).

    Signed-off-by: Neil Horman
    Signed-off-by: Patrick McHardy
    Signed-off-by: David S. Miller

    Neil Horman
     

13 Feb, 2007

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01 Jul, 2006

1 commit

21 Mar, 2006

3 commits

30 Aug, 2005

1 commit