linux-kernel - Re: [PATCH] aio: fix a user triggered use after free (and fix freeze protection of aio writes)

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Message-ID: <20161029122451.GQ19539@ZenIV.linux.org.uk>
Date:   Sat, 29 Oct 2016 13:24:51 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     torvalds@...ux-foundation.org, jack@...e.cz, dmonakhov@...nvz.org,
        jmoyer@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-aio@...ck.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] aio: fix a user triggered use after free (and fix freeze
 protection of aio writes)

On Sat, Oct 29, 2016 at 09:44:29AM +0200, Christoph Hellwig wrote:

> -		if (rw == WRITE)
> +		if (rw == WRITE) {
>  			file_start_write(file);
> +			req->ki_flags |= IOCB_WRITE;
> +		}

> +		if (rw == WRITE) {
> +			/*
> +			 * We release freeze protection in aio_complete(). Fool
> +			 * lockdep by telling it the lock got released so that
> +			 * it doesn't complain about held lock when we return
> +			 * to userspace.
> +			 */
> +			__sb_writers_release(file_inode(file)->i_sb,
> +					SB_FREEZE_WRITE);
> +		}

How about taking this chunk (i.e. telling lockdep that we are not holding this
thing) past the iter_op() call, where file_end_write() used to be?

As it is, you risk hiding the lock dependencies the current mainline would've
caught.  Other than that I see no problems with the patch...