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

[Rik van Riel]

On 09/03/2013 10:00 AM, Manfred Spraul 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.
> - 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.
> 
> Observed with msgctl08 from ltp with a preemptible kernel.
> 
> 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>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>


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