[Devel] [PATCH v2] nfs: protect callback execution against per-net callback thread shutdown

[Stanislav Kinsburskiy]

Sorry, this one is bad as well.

07.11.2017 11:28, Stanislav Kinsburskiy пишет:
> Here is the race:
> 
> CPU #0				CPU#1
> 
> cleanup_mnt			nfs41_callback_svc (get xprt from the list)
> nfs_callback_down		...
> ...				...
> svc_close_net			...
> ...				...
> svc_xprt_free			...
> svc_bc_sock_free		bc_svc_process
> kfree(xprt)			svc_process_common
> 				rqstp->rq_xprt->xpt_ops (use after free)
> 
> The problem is that per-net SUNRPC transports shutdown is done regardless
> current callback execution. This is a race leading to transport use-after-free
> in callback handler.
> This patch fixes it in stright-forward way. I.e. it protects callback
> execution with the same mutex used for per-net data creation and destruction.
> Hopefully, it won't slow down NFS client significantly.
> 
> https://jira.sw.ru/browse/PSBM-75751
> 
> v2: Fix mutex deadlock, when shutdown callback waits for thread to exit (with
> mutex taken), while thread wait for the mutex to take.
> 
> Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
> ---
>  fs/nfs/callback.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> index 0beb275..eed861a 100644
> --- a/fs/nfs/callback.c
> +++ b/fs/nfs/callback.c
> @@ -118,6 +118,11 @@ nfs41_callback_svc(void *vrqstp)
>  			continue;
>  
>  		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_INTERRUPTIBLE);
> +
> +		if (!mutex_trylock(&nfs_callback_mutex) &&
> +		    kthread_should_stop())
> +			return 0;
> +
>  		spin_lock_bh(&serv->sv_cb_lock);
>  		if (!list_empty(&serv->sv_cb_list)) {
>  			req = list_first_entry(&serv->sv_cb_list,
> @@ -129,8 +134,10 @@ nfs41_callback_svc(void *vrqstp)
>  			error = bc_svc_process(serv, req, rqstp);
>  			dprintk("bc_svc_process() returned w/ error code= %d\n",
>  				error);
> +			mutex_unlock(&nfs_callback_mutex);
>  		} else {
>  			spin_unlock_bh(&serv->sv_cb_lock);
> +			mutex_unlock(&nfs_callback_mutex);
>  			schedule();
>  			finish_wait(&serv->sv_cb_waitq, &wq);
>  		}
> 
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