linux-kernel - Re: [patch 3/5] fs: fix bdi writeback use after free 1

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Message-ID: <20090915193204.GU23126@kernel.dk>
Date:	Tue, 15 Sep 2009 21:32:04 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	npiggin@...e.de
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 3/5] fs: fix bdi writeback use after free 1

On Wed, Sep 16 2009, npiggin@...e.de wrote:
> By the time bdi_work_on_stack gets evaluated again in bdi_work_free, it
> cna already have been deallocated and used for something else in the
> !on stack case, giving a false positive in this test and causing corruption.

You are right, that is also buggy... Applied.

> 
> Signed-off-by: Nick Piggin <npiggin@...e.de>
> ---
>  fs/fs-writeback.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c
> +++ linux-2.6/fs/fs-writeback.c
> @@ -114,6 +114,7 @@ static void bdi_work_free(struct rcu_hea
>  static void wb_work_complete(struct bdi_work *work)
>  {
>  	const enum writeback_sync_modes sync_mode = work->sync_mode;
> +	int onstack = bdi_work_on_stack(work);
>  
>  	/*
>  	 * For allocated work, we can clear the done/seen bit right here.
> @@ -121,9 +122,9 @@ static void wb_work_complete(struct bdi_
>  	 * to after the RCU grace period, since the stack could be invalidated
>  	 * as soon as bdi_work_clear() has done the wakeup.
>  	 */
> -	if (!bdi_work_on_stack(work))
> +	if (!onstack)
>  		bdi_work_clear(work);
> -	if (sync_mode == WB_SYNC_NONE || bdi_work_on_stack(work))
> +	if (sync_mode == WB_SYNC_NONE || onstack)
>  		call_rcu(&work->rcu_head, bdi_work_free);
>  }
>  
> 
> 

-- 
Jens Axboe

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