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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Christoph Hellwig <hch@lst.de>
Cc: Avi Kivity <avi@scylladb.com>,
	linux-aio@kvack.org, linux-fsdevel@vger.kernel.org,
	netdev@vger.kernel.org, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/31] aio: implement IOCB_CMD_POLL
Date: Wed, 23 May 2018 01:45:30 +0100	[thread overview]
Message-ID: <20180523004530.GG30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180522220524.GE30522@ZenIV.linux.org.uk>

On Tue, May 22, 2018 at 11:05:24PM +0100, Al Viro wrote:
> > +{
> > +	struct aio_kiocb *iocb = container_of(req, struct aio_kiocb, poll);
> > +
> > +	fput(req->file);
> > +	aio_complete(iocb, mangle_poll(mask), 0);
> > +}
> 
> Careful.
> 
> > +static int aio_poll_cancel(struct kiocb *iocb)
> > +{
> > +	struct aio_kiocb *aiocb = container_of(iocb, struct aio_kiocb, rw);
> > +	struct poll_iocb *req = &aiocb->poll;
> > +	struct wait_queue_head *head = req->head;
> > +	bool found = false;
> > +
> > +	spin_lock(&head->lock);
> > +	found = __aio_poll_remove(req);
> > +	spin_unlock(&head->lock);
> 
> What's to guarantee that req->head has not been freed by that point?
> Look: wakeup finds ->ctx_lock held, so it leaves the sucker on the
> list, removes it from queue and schedules the call of __aio_poll_complete().
> Which gets executed just as we hit aio_poll_cancel(), starting with fput().
> 
> You really want to do aio_complete() before fput().  That way you know that
> req->wait is alive and well at least until iocb gets removed from the list.

Oh, bugger...

wakeup
	removed from queue
	schedule __aio_poll_complete()

cancel
	grab ctx->lock
	remove from list
work
	aio_complete()
		check if it's in the list
		it isn't, move on to free the sucker
cancel
	call ->ki_cancel()
	BOOM

Looks like we want to call ->ki_cancel() *BEFORE* removing from the list,
as well as doing fput() after aio_complete().  The same ordering, BTW, goes
for aio_read() et.al.

Look:
CPU1:	io_cancel() grabs ->ctx_lock, finds iocb and removes it from the list.
CPU2:	aio_rw_complete() on that iocb.  Since the sucker is not in the list
anymore, we do NOT spin on ->ctx_lock and proceed to free iocb
CPU1:	pass freed iocb to ->ki_cancel().  BOOM.

and if we have fput() done first (in aio_rw_complete()) we are vulnerable to
CPU1:	io_cancel() grabs ->ctx_lock, finds iocb and removes it from the list.
CPU2:	aio_rw_complete() on that iocb. fput() done, opening us to rmmod.
CPU1:	call ->ki_cancel(), which points to freed memory now.  BOOM.

  reply	other threads:[~2018-05-23  0:45 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-22 11:30 aio poll and a new in-kernel poll API V12 Christoph Hellwig
2018-05-22 11:30 ` [PATCH 01/31] fs: unexport poll_schedule_timeout Christoph Hellwig
2018-05-22 11:30 ` [PATCH 02/31] fs: cleanup do_pollfd Christoph Hellwig
2018-05-22 11:30 ` [PATCH 03/31] fs: update documentation to mention __poll_t and match the code Christoph Hellwig
2018-05-22 11:30 ` [PATCH 04/31] fs: add new vfs_poll and file_can_poll helpers Christoph Hellwig
2018-05-22 11:30 ` [PATCH 05/31] fs: introduce new ->get_poll_head and ->poll_mask methods Christoph Hellwig
2018-05-22 11:30 ` [PATCH 06/31] aio: simplify KIOCB_KEY handling Christoph Hellwig
2018-05-22 11:30 ` [PATCH 07/31] aio: simplify cancellation Christoph Hellwig
2018-05-22 11:30 ` [PATCH 08/31] aio: implement IOCB_CMD_POLL Christoph Hellwig
2018-05-22 22:05   ` Al Viro
2018-05-23  0:45     ` Al Viro [this message]
2018-05-23  0:49       ` Al Viro
2018-05-23  1:43         ` YAaioRace (was Re: [PATCH 08/31] aio: implement IOCB_CMD_POLL) Al Viro
2018-05-22 11:30 ` [PATCH 09/31] aio: try to complete poll iocbs without context switch Christoph Hellwig
2018-05-22 11:30 ` [PATCH 10/31] net: refactor socket_poll Christoph Hellwig
2018-05-22 11:30 ` [PATCH 11/31] net: add support for ->poll_mask in proto_ops Christoph Hellwig
2018-05-22 11:30 ` [PATCH 12/31] net: remove sock_no_poll Christoph Hellwig
2018-05-22 11:30 ` [PATCH 13/31] net/tcp: convert to ->poll_mask Christoph Hellwig
2018-05-22 11:30 ` [PATCH 14/31] net/unix: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 15/31] net: convert datagram_poll users tp ->poll_mask Christoph Hellwig
2018-05-22 11:30 ` [PATCH 16/31] net/dccp: convert to ->poll_mask Christoph Hellwig
2018-05-22 11:30 ` [PATCH 17/31] net/atm: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 18/31] net/vmw_vsock: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 19/31] net/tipc: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 20/31] net/sctp: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 21/31] net/bluetooth: " Christoph Hellwig
2018-05-22 11:30 ` [PATCH 22/31] net/caif: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 23/31] net/nfc: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 24/31] net/phonet: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 25/31] net/iucv: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 26/31] net/rxrpc: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 27/31] crypto: af_alg: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 28/31] pipe: " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 29/31] eventfd: switch " Christoph Hellwig
2018-05-22 11:31 ` [PATCH 30/31] timerfd: convert " Christoph Hellwig
2018-05-22 16:59   ` Sergei Shtylyov
2018-05-22 11:31 ` [PATCH 31/31] random: " Christoph Hellwig
2018-05-22 22:07 ` aio poll and a new in-kernel poll API V12 Al Viro

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