netdev - Re: [PATCH net-next] rhashtable: rhashtable_remove() must unlink in both tbl and future

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Date:	Wed, 21 Jan 2015 16:51:05 +0800
From:	Ying Xue <ying.xue@...driver.com>
To:	Thomas Graf <tgraf@...g.ch>,
	"richard.alpe@...csson.com >> Richard Alpe" 
	<richard.alpe@...csson.com>
CC:	<davem@...emloft.net>, Netdev <netdev@...r.kernel.org>,
	<tipc-discussion@...ts.sourceforge.net>
Subject: Re: [PATCH net-next] rhashtable: rhashtable_remove() must unlink
 in both tbl and future_tbl

Hi Thomas,

I will be on vacation in next few days. So if you have an updated patch
resolving the issue, please let Richard(richard.alpe@...csson.com) help
you to verify because he initially found the issue. After I identified
what it closely was related to rhashtable and it also happened in
netlink, I posted it on netdev mail list.

Regards,
Ying

On 01/21/2015 12:58 AM, Thomas Graf wrote:
> As removals can occur during resizes, entries may be referred to from
> both tbl and future_tbl when the removal is requested. Therefore
> rhashtable_remove() must unlink the entry in both tables if this is
> the case. The existing code did search both tables but stopped when it
> hit the first match.
> 
> Failing to do so resulted in use after remove.
> 
> Fixes: 97defe1 ("rhashtable: Per bucket locks & deferred expansion/shrinking")
> Reported-by: Ying Xue <ying.xue@...driver.com>
> Signed-off-by: Thomas Graf <tgraf@...g.ch>
> ---
> 
> Ying: This should fix the panic that was at the end of your TIPC
>       related boot log. I'm still working on the use after free.
> 
>  lib/rhashtable.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index a4449c4..b1aa10e 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -592,6 +592,7 @@ bool rhashtable_remove(struct rhashtable *ht, struct rhash_head *obj)
>  	struct rhash_head *he;
>  	spinlock_t *lock;
>  	unsigned int hash;
> +	bool ret = false;
>  
>  	rcu_read_lock();
>  	tbl = rht_dereference_rcu(ht->tbl, ht);
> @@ -609,17 +610,16 @@ restart:
>  		}
>  
>  		rcu_assign_pointer(*pprev, obj->next);
> -		atomic_dec(&ht->nelems);
> -
> -		spin_unlock_bh(lock);
> -
> -		rhashtable_wakeup_worker(ht);
> -
> -		rcu_read_unlock();
>  
> -		return true;
> +		ret = true;
> +		break;
>  	}
>  
> +	/* The entry may be linked in either 'tbl', 'future_tbl', or both.
> +	 * 'future_tbl' only exists for a short period of time during
> +	 * resizing. Thus traversing both is fine and the added cost is
> +	 * very rare.
> +	 */
>  	if (tbl != rht_dereference_rcu(ht->future_tbl, ht)) {
>  		spin_unlock_bh(lock);
>  
> @@ -632,9 +632,15 @@ restart:
>  	}
>  
>  	spin_unlock_bh(lock);
> +
> +	if (ret) {
> +		atomic_dec(&ht->nelems);
> +		rhashtable_wakeup_worker(ht);
> +	}
> +
>  	rcu_read_unlock();
>  
> -	return false;
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(rhashtable_remove);
>  
> 

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