linux-kernel - Re: [PATCH V1] fuse: Set fuse request error upon fuse abort connection

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Date:   Mon, 21 Jun 2021 09:48:45 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Pradeep P V K <pragalla@...eaurora.org>
Cc:     stummala@...eaurora.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V1] fuse: Set fuse request error upon fuse abort
 connection

On Fri, Apr 16, 2021 at 07:49:12PM +0530, Pradeep P V K wrote:
> There is a minor race in setting the fuse out request error
> between fuse_abort_conn() and fuse_dev_do_read() as explained
> below.
> 
> Thread-1			  Thread-2
> ========			  ========
> ->fuse_simple_request()           ->shutdown
>   ->__fuse_request_send()
>     ->queue_request()		->fuse_abort_conn()
> ->fuse_dev_do_read()                ->acquire(fpq->lock)
>   ->wait_for(fpq->lock) 	  ->set err to all req's in fpq->io
> 				  ->release(fpq->lock)
>   ->acquire(fpq->lock)
>   ->add req to fpq->io
> 
> The above scenario may cause Thread-1 request to add into
> fpq->io list after Thread-2 sets -ECONNABORTED err to all
> its requests in fpq->io list. This leaves Thread-1 request
> with unset err and this further misleads as a completed
> request without an err set upon request_end().
> 
> Handle this by setting the err appropriately.

The fix looks good, but still allows the request to block during the copy phase
after being aborted, which the FR_LOCKED/FR_ABORT bits are meant to prevent.

Here's an updated fix.  It does not allow the request to be queued on the
fpq->io list after fuse_abort_conn() has aborted requests on that list.

Can you verify that it fixes the race you reported?

Thanks,
Miklos


--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1272,6 +1272,15 @@ static ssize_t fuse_dev_do_read(struct f
 		goto restart;
 	}
 	spin_lock(&fpq->lock);
+	/*
+	 *  Must not put request on fpq->io queue after having been shut down by
+	 *  fuse_abort_conn()
+	 */
+	if (!fpq->connected) {
+		req->out.h.error = err = -ECONNABORTED;
+		goto out_end;
+
+	}
 	list_add(&req->list, &fpq->io);
 	spin_unlock(&fpq->lock);
 	cs->req = req;