Re: [PATCH] ipc/msg.c: Fix lost wakeup in msgsnd().

[Sedat Dilek <sedat.dilek@gmail.com>]

On Tue, Sep 3, 2013 at 4:00 PM, Manfred Spraul <manfred@colorfullife.com> wrote:
> The check if the queue is full and adding current to the wait queue of pending
> msgsnd() operations (ss_add()) must be atomic.
>
> Otherwise:
> - the thread that performs msgsnd() finds a full queue and decides to sleep.
> - the thread that performs msgrcv() calls first reads all messages from the
>   queue and then sleep, because the queue is empty.

reads -> sleeps

> - the msgrcv() calls do not perform any wakeups, because the msgsnd() task
>   has not yet called ss_add().
> - then the msgsnd()-thread first calls ss_add() and then sleeps.
> Net result: msgsnd() and msgrcv() both sleep forever.
>

I don't know what and why "net result" - net in sense of networking?

> Observed with msgctl08 from ltp with a preemptible kernel.
>

...on ARC arch (that sounds funny somehow).

> Fix: Call ipc_lock_object() before performing the check.
>
> The patch also moves security_msg_queue_msgsnd() under ipc_lock_object:
> - msgctl(IPC_SET) explicitely mentions that it tries to expunge any pending
>   operations that are not allowed anymore with the new permissions.
>   If security_msg_queue_msgsnd() is called without locks, then there might be
>   races.
> - it makes the patch much simpler.
>
> Reported-by: Vineet Gupta <Vineet.Gupta1@synopsys.com>
> Signed-off-by: Manfred Spraul <manfred@colorfullife.com>

I guess this is missing a "CC: stable" as Vineet reported against
Linux v3.11-rc7 (and should enter v3.11.1)?

- Sedat -

> ---
>  ipc/msg.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 9f29d9e..b65fdf1 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -680,16 +680,18 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
>                 goto out_unlock1;
>         }
>
> +       ipc_lock_object(&msq->q_perm);
> +
>         for (;;) {
>                 struct msg_sender s;
>
>                 err = -EACCES;
>                 if (ipcperms(ns, &msq->q_perm, S_IWUGO))
> -                       goto out_unlock1;
> +                       goto out_unlock0;
>
>                 err = security_msg_queue_msgsnd(msq, msg, msgflg);
>                 if (err)
> -                       goto out_unlock1;
> +                       goto out_unlock0;
>
>                 if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
>                                 1 + msq->q_qnum <= msq->q_qbytes) {
> @@ -699,10 +701,9 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
>                 /* queue full, wait: */
>                 if (msgflg & IPC_NOWAIT) {
>                         err = -EAGAIN;
> -                       goto out_unlock1;
> +                       goto out_unlock0;
>                 }
>
> -               ipc_lock_object(&msq->q_perm);
>                 ss_add(msq, &s);
>
>                 if (!ipc_rcu_getref(msq)) {
> @@ -730,10 +731,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
>                         goto out_unlock0;
>                 }
>
> -               ipc_unlock_object(&msq->q_perm);
>         }
> -
> -       ipc_lock_object(&msq->q_perm);
>         msq->q_lspid = task_tgid_vnr(current);
>         msq->q_stime = get_seconds();
>
> --
> 1.8.3.1
>

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