linux-kernel - Re: [PATCH v7 10/10] lib/dlock-list: Fix use-after-unlock problem in dlist_for_each_entry

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Date:   Mon, 30 Oct 2017 10:06:40 +0100
From:   Jan Kara <jack@...e.cz>
To:     Waiman Long <longman@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v7 10/10] lib/dlock-list: Fix use-after-unlock problem in
 dlist_for_each_entry_safe()

On Fri 27-10-17 16:10:53, Waiman Long wrote:
> The dlist_for_each_entry_safe() macro in include/linux/dlock-list has
> a use-after-unlock problem where racing condition can happen because
> of a lack of spinlock protection.  Fortunately, this macro is not
> currently being used in the kernel.
> 
> This patch changes the dlist_for_each_entry_safe() macro so that the
> call to __dlock_list_next_list() is deferred until the next entry is
> being used. That should eliminate the use-after-unlock problem.
> 
> Reported-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Waiman Long <longman@...hat.com>

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza


> ---
>  include/linux/dlock-list.h | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
> index 02c5f4d..f4b7657 100644
> --- a/include/linux/dlock-list.h
> +++ b/include/linux/dlock-list.h
> @@ -191,17 +191,17 @@ extern void dlock_list_add(struct dlock_list_node *node,
>  }
>  
>  /**
> - * dlock_list_first_entry - get the first element from a list
> + * dlock_list_next_list_entry - get first element from next list in iterator
>   * @iter  : The dlock list iterator.
> - * @type  : The type of the struct this is embedded in.
> + * @pos   : A variable of the struct that is embedded in.
>   * @member: The name of the dlock_list_node within the struct.
> - * Return : Pointer to the next entry or NULL if all the entries are iterated.
> + * Return : Pointer to first entry or NULL if all the lists are iterated.
>   */
> -#define dlock_list_first_entry(iter, type, member)			\
> +#define dlock_list_next_list_entry(iter, pos, member)			\
>  	({								\
>  		struct dlock_list_node *_n;				\
>  		_n = __dlock_list_next_entry(NULL, iter);		\
> -		_n ? list_entry(_n, type, member) : NULL;		\
> +		_n ? list_entry(_n, typeof(*pos), member) : NULL;	\
>  	})
>  
>  /**
> @@ -231,7 +231,7 @@ extern void dlock_list_add(struct dlock_list_node *node,
>   * This iteration function is designed to be used in a while loop.
>   */
>  #define dlist_for_each_entry(pos, iter, member)				\
> -	for (pos = dlock_list_first_entry(iter, typeof(*(pos)), member);\
> +	for (pos = dlock_list_next_list_entry(iter, pos, member);	\
>  	     pos != NULL;						\
>  	     pos = dlock_list_next_entry(pos, iter, member))
>  
> @@ -245,14 +245,20 @@ extern void dlock_list_add(struct dlock_list_node *node,
>   * This iteration macro is safe with respect to list entry removal.
>   * However, it cannot correctly iterate newly added entries right after the
>   * current one.
> + *
> + * The call to __dlock_list_next_list() is deferred until the next entry
> + * is being iterated to avoid use-after-unlock problem.
>   */
>  #define dlist_for_each_entry_safe(pos, n, iter, member)			\
> -	for (pos = dlock_list_first_entry(iter, typeof(*(pos)), member);\
> +	for (pos = NULL;						\
>  	    ({								\
> -		bool _b = (pos != NULL);				\
> -		if (_b)							\
> -			n = dlock_list_next_entry(pos, iter, member);	\
> -		_b;							\
> +		if (!pos ||						\
> +		   (&(pos)->member.list == &(iter)->entry->list))	\
> +			pos = dlock_list_next_list_entry(iter, pos,	\
> +							 member);	\
> +		if (pos)						\
> +			n = list_next_entry(pos, member.list);		\
> +		pos;							\
>  	    });								\
>  	    pos = n)
>  
> -- 
> 1.8.3.1
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR