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

[Sedat Dilek]

On Tue, Sep 3, 2013 at 6:33 PM, Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
> Hi Sedat,
>
>
> On 09/03/2013 06:13 PM, Sedat Dilek wrote:
>>
>> On Tue, Sep 3, 2013 at 4:00 PM, Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
>> 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
>
> Correct.
>
>>> - 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?
>
> http://en.wiktionary.org/wiki/net#Adjective
> I.e.: Ignore/remove the "Net".
>
>
>>> 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@xxxxxxxxxxxx>
>>> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
>>
>> I guess this is missing a "CC: stable" as Vineet reported against
>> Linux v3.11-rc7 (and should enter v3.11.1)?
>
> Yes. I didn't notice that Linus already released 3.11.
>

Linus pushed your patch upstream... with typos fixed and "CC: stable #3.11".
Thanks to all involved people!

- Sedat -

[1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=bebcb928c820d0ee83aca4b192adc195e43e66a2

> --
>     Manfred
>
>> - 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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