Re: [PATCH v7 10/10] lib/dlock-list: Fix use-after-unlock problem in dlist_for_each_entry_safe()

[Davidlohr Bueso]

On Fri, 27 Oct 2017, 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@xxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>

But would it not be better to merge this patch (among others) into 1/N?
Specifically the newer patches 7-10 should be in the original dlock
implementation instead of adding fixes to incorrect code in the original
commit. Also less of a pita for backporting.

Thanks,
Davidlohr


> ---
> 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