Re: dm mpath: add check for count of groups to avoid wild pointer access

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Nov 03 2016 at  6:49am -0400,
tang.junhui@zte.com.cn <tang.junhui@zte.com.cn> wrote:

> From: "tang.junhui" <tang.junhui@zte.com.cn>
> 
> pg is not assigned to a group address when count of multipath groups
> is zero in bypass_pg_num(), then it is used in bypass_pg(), which may
> cause wild pointer access.
> 
> Signed-off-by: tang.junhui <tang.junhui@zte.com.cn>
> ---
>  drivers/md/dm-mpath.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> index d376dc8..8c1359c 100644
> --- a/drivers/md/dm-mpath.c
> +++ b/drivers/md/dm-mpath.c
> @@ -1084,7 +1084,7 @@ static int switch_pg_num(struct multipath *m, const char *pgstr)
>  	char dummy;
>  
>  	if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 1) || !pgnum ||
> -	    (pgnum > m->nr_priority_groups)) {
> +	    !m->nr_priority_groups || (pgnum > m->nr_priority_groups)) {
>  		DMWARN("invalid PG number supplied to switch_pg_num");
>  		return -EINVAL;
>  	}
> -- 
> 2.8.1.windows.1
> 
> 

You mention bypass_pg_num() going on to hit a NULL/"wild" pointer.  Not
immediately seeing the relation between switch_pg_num() and
bypass_pg_num().  But shouldn't bypass_pg_num() have improved bounds
checking (and/or NULL pointer checks) too?

Maybe your patch was applied with an offset and it modified
switch_pg_num() when you really meant to modify bypass_pg_num()?