Re: [PATCH v3 1/9] driver core: class: Fix wild pointer dereferences in API class_dev_iter_next()

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Thu, 12 Dec 2024 21:38:37 +0800
Zijun Hu <zijun_hu@icloud.com> wrote:

> From: Zijun Hu <quic_zijuhu@quicinc.com>
> 
> There are a potential wild pointer dereferences issue regarding APIs
> class_dev_iter_(init|next|exit)(), as explained by below typical usage:
> 
> // All members of @iter are wild pointers.
> struct class_dev_iter iter;
> 
> // class_dev_iter_init(@iter, @class, ...) checks parameter @class for
> // potential class_to_subsys() error, and it returns void type and does
> // not initialize its output parameter @iter, so caller can not detect
> // the error and continues to invoke class_dev_iter_next(@iter) even if
> // @iter still contains wild pointers.
> class_dev_iter_init(&iter, ...);
> 
> // Dereference these wild pointers in @iter here once suffer the error.
> while (dev = class_dev_iter_next(&iter)) { ... };
> 
> // Also dereference these wild pointers here.
> class_dev_iter_exit(&iter);
> 
> Actually, all callers of these APIs have such usage pattern in kernel tree.
> Fix by:
> - Initialize output parameter @iter by memset() in class_dev_iter_init()
>   and give callers prompt by pr_crit() for the error.
> - Check if @iter is valid in class_dev_iter_next().
> 
> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>

This looks fine in general, but over to the core device model folk for which
element they think should be used as the sentinel and whether zeroing the
whole thing makes sense or just the one being used as a flag, or even setting
it to an error pointer.

> 
> ---
> Alternative fix solutions ever thought about:
> 
> 1) Use BUG_ON(!sp) instead of error return in class_dev_iter_init().
> 2) Change class_dev_iter_init()'s type to int, lots of jobs to do.
> 
> This issue is APIs themself issues, and regardless of how various API
> users use them, and silent wild pointer dereferences are not what API
> users expect for the error absolutely.
> ---
>  drivers/base/class.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/class.c b/drivers/base/class.c
> index 582b5a02a5c410113326601fe00eb6d7231f988f..d57f277978dc9033fba3484b4620bcf884a4029f 100644
> --- a/drivers/base/class.c
> +++ b/drivers/base/class.c
> @@ -323,8 +323,12 @@ void class_dev_iter_init(struct class_dev_iter *iter, const struct class *class,
>  	struct subsys_private *sp = class_to_subsys(class);
>  	struct klist_node *start_knode = NULL;
>  
> -	if (!sp)
> +	memset(iter, 0, sizeof(*iter));
> +	if (!sp) {
> +		pr_crit("%s: class %p was not registered yet\n",
> +			__func__, class);
>  		return;
> +	}
>  
>  	if (start)
>  		start_knode = &start->p->knode_class;
> @@ -351,6 +355,9 @@ struct device *class_dev_iter_next(struct class_dev_iter *iter)
>  	struct klist_node *knode;
>  	struct device *dev;
>  
> +	if (!iter->sp)
> +		return NULL;
> +
>  	while (1) {
>  		knode = klist_next(&iter->ki);
>  		if (!knode)
>