Re: [PATCH] ipc/mqueue: Avoid relying on a stack reference past its expiry

[Manfred Spraul]

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Hi Varad,

On 5/4/21 5:55 PM, Varad Gautam wrote:
> do_mq_timedsend::__pipelined_op() should not dereference `this` after
> setting STATE_READY, as the receiver counterpart is now free to return.
> Change __pipelined_op to call wake_q_add_safe on the receiver's
> task_struct returned by get_task_struct, instead of dereferencing
> `this` which sits on the receiver's stack.
Correct. I was so concentrated on the risks of reordered memory that I 
have overlooked the simple bug.
> Fixes: c5b2cbdbdac563 ("ipc/mqueue.c: update/document memory barriers")
Actually, sem.c and msg.c contain the same bug. Thus all three must be 
fixed.
> Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
> Reported-by: Matthias von Faber <matthias.vonfaber@xxxxxxxxxxx>
> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dbueso@xxxxxxx>
>
> ---
>   ipc/mqueue.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/mqueue.c b/ipc/mqueue.c
> index 8031464ed4ae2..8f78057c6be53 100644
> --- a/ipc/mqueue.c
> +++ b/ipc/mqueue.c
> @@ -1004,12 +1004,14 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
>   				  struct mqueue_inode_info *info,
>   				  struct ext_wait_queue *this)
>   {
> +	struct task_struct *t;
> +
>   	list_del(&this->list);
> -	get_task_struct(this->task);
> +	t = get_task_struct(this->task);
>   
>   	/* see MQ_BARRIER for purpose/pairing */
>   	smp_store_release(&this->state, STATE_READY);
> -	wake_q_add_safe(wake_q, this->task);
> +	wake_q_add_safe(wake_q, t);
>   }

The change fixes the issue, but I would prefer to use t = this->task 
instead of using the return value of get_task_struct():
Then all wake_q_add_safe() users are identical.

Ok for you?

Slightly tested patch attached.

--

    Manfred