Re: [PATCH] swapfile: avoid NULL pointer dereference in swapon whens_bdev is NULL

[Hugh Dickins]

On Tue, 29 Sep 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@xxxxxxx>

Acked-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>

Thanks a lot for that: sorry to say I was ignorant of the possibility.

This is only an issue with an out-of-tree patch, is that correct?
I'd like it to be fixed anyway, but if there's a way in which it can
happen in unpatched 2.6.31, then we ought to send the fix to -stable.

I've added Rafael to the Cc, because CONFIG_HIBERNATION's swap_type_of()
looks also dangerous in this respect - and especially where it does that
"if (bdev == sis->bdev) {" match, I think it's assuming NULL bdev cannot
match against anything.

Hugh

> ---
>  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);
--
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