Re: dm-mpath: Fix setup_scsi_dh()

[Bart Van Assche <bvanassche@acm.org>]

On 9/17/18 7:20 AM, Mike Snitzer wrote:
>> - Avoid that m->hw_handler_name becomes a dangling pointer if the
>>    RETAIN_ATTACHED_HW_HANDLER flag is set and scsi_dh_attach() returns
>>    -EBUSY.
> 
> What is the concern about a dangling pointer?  How does that manifest?
> Stale scsi_dh name stored in hw_handler_name?  Pretty sure it gets freed
> and reassigned as needed (at the start of setup_scsi_dh).

Hello Mike,

Thanks for having taken a look. Before commit e8f74a0f0011, if both 
MPATHF_RETAIN_ATTACHED_HW_HANDLER and m->hw_handler_name are set before 
setup_scsi_dh() is called and if scsi_dh_attach() returns -EBUSY, 
scsi_dh_attached_handler_name() was called twice and allocated memory 
twice for the handler name. Since commit e8f74a0f0011, in that scenario, 
the following code related to the handler name is executed:

	kfree(m->hw_handler_name);
	m->hw_handler_name = attached_handler_name;
	[ scsi_dh_attach() returns -EBUSY ]
	kfree(m->hw_handler_name);
	m->hw_handler_name = attached_handler_name;

I think this sequence makes m->hw_handler_name a dangling pointer.

> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> index d94ba6f72ff5..688ac9e719a7 100644
> --- a/drivers/md/dm-mpath.c
> +++ b/drivers/md/dm-mpath.c
> @@ -806,14 +806,14 @@ static int parse_path_selector(struct dm_arg_set *as, struct priority_group *pg,
>   }
>   
>   static int setup_scsi_dh(struct block_device *bdev, struct multipath *m,
> -			 const char *attached_handler_name, char **error)
> +			 char **attached_handler_name, char **error)
>   {
>   	struct request_queue *q = bdev_get_queue(bdev);
>   	int r;
>   
>   	if (test_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags)) {
>   retain:
> -		if (attached_handler_name) {
> +		if (*attached_handler_name) {
>   			/*
>   			 * Clear any hw_handler_params associated with a
>   			 * handler that isn't already attached.
> @@ -830,7 +830,8 @@ static int setup_scsi_dh(struct block_device *bdev, struct multipath *m,
>   			 * handler instead of the original table passed in.
>   			 */
>   			kfree(m->hw_handler_name);
> -			m->hw_handler_name = attached_handler_name;
> +			m->hw_handler_name = *attached_handler_name;
> +			*attached_handler_name = NULL;
>   		}
>   	}
>   
> @@ -867,7 +868,7 @@ static struct pgpath *parse_path(struct dm_arg_set *as, struct path_selector *ps
>   	struct pgpath *p;
>   	struct multipath *m = ti->private;
>   	struct request_queue *q;
> -	const char *attached_handler_name;
> +	char *attached_handler_name = NULL;
>   
>   	/* we need at least a path arg */
>   	if (as->argc < 1) {
> @@ -890,7 +891,7 @@ static struct pgpath *parse_path(struct dm_arg_set *as, struct path_selector *ps
>   	attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL);
>   	if (attached_handler_name || m->hw_handler_name) {
>   		INIT_DELAYED_WORK(&p->activate_path, activate_path_work);
> -		r = setup_scsi_dh(p->path.dev->bdev, m, attached_handler_name, &ti->error);
> +		r = setup_scsi_dh(p->path.dev->bdev, m, &attached_handler_name, &ti->error);
>   		if (r) {
>   			dm_put_device(ti, p->path.dev);
>   			goto bad;
> @@ -905,6 +906,8 @@ static struct pgpath *parse_path(struct dm_arg_set *as, struct path_selector *ps
>   
>   	return p;
>    bad:
> +	if (attached_handler_name)
> +		kfree(attached_handler_name);
>   	free_pgpath(p);
>   	return ERR_PTR(r);
>   }

Except that the if (attached_handler_name) should be removed from before 
the kfree() call, the above looks good to me. But since we can avoid 
changing the type of attached_handler_name from char * into char ** by 
moving the kfree() call into setup_scsi_dh(), I prefer to avoid to make 
that change.

Thanks,

Bart.