linux-kernel - Re: [PATCH] swapfile: avoid NULL pointer dereference in swapon when s

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Date:	Wed, 30 Sep 2009 08:57:22 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Suresh Jayaraman <sjayaraman@...e.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] swapfile: avoid NULL pointer dereference in swapon
	when s_bdev  is NULL

On Tue, Sep 29 2009, Suresh Jayaraman wrote:
> While testing Swap over NFS patchset, I noticed an oops that was triggered
> during swapon. Investigating further, the NULL pointer deference is due to the
> SSD device check/optimization in the swapon code that assumes s_bdev could never
> be NULL.
> 
> inode->i_sb->s_bdev could be NULL in a few cases. For e.g. one such case is
> loopback NFS mount, there could be others as well. Fix this by ensuring s_bdev
> is not NULL before we try to deference s_bdev.
> 
> Signed-off-by: Suresh Jayaraman <sjayaraman@...e.de>
> ---
>  mm/swapfile.c |   12 +++++++-----
>  1 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4de7f02..a1bc6b9 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1974,12 +1974,14 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		goto bad_swap;
>  	}
>  
> -	if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
> -		p->flags |= SWP_SOLIDSTATE;
> -		p->cluster_next = 1 + (random32() % p->highest_bit);
> +	if (p->bdev) {
> +		if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
> +			p->flags |= SWP_SOLIDSTATE;
> +			p->cluster_next = 1 + (random32() % p->highest_bit);
> +		}
> +		if (discard_swap(p) == 0)
> +			p->flags |= SWP_DISCARDABLE;
>  	}
> -	if (discard_swap(p) == 0)
> -		p->flags |= SWP_DISCARDABLE;
>  
>  	mutex_lock(&swapon_mutex);
>  	spin_lock(&swap_lock);

Thanks for the patch, looks correct.

-- 
Jens Axboe

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