Re: [PATCH] net-sysfs: fix NULL pointer dereference

[Antoine Tenart <atenart@kernel.org>]

Quoting Qasim Ijaz (2025-03-06 00:53:07)
> Commit <79c61899b5ee> introduces a potential NULL pointer dereference 
> in the sysfs_rtnl_lock() function when initialising kn:
> 
>         kn = sysfs_break_active_protection(kobj, attr);
>         
> The commit overlooks the fact that sysfs_break_active_protection can 
> return NULL if kernfs_find_and_get() fails to find and get the kernfs_node 
> with the given name. 

If it fails to get it, should we let sysfs_rtnl_lock continue is
execution?

> Later on the code calls sysfs_unbreak_active_protection(kn) 
> unconditionally, which could lead to a NULL pointer dereference.
> 
> Resolve this bug by introducing a NULL check before using kn
> in the sysfs_unbreak_active_protection() call.

Did you see this in practice? Can you describe what led to this?

Thanks!
Antoine

> Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
> Fixes: 79c61899b5ee ("net-sysfs: remove rtnl_trylock from device attributes")
> ---
>  net/core/net-sysfs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 8d9dc048a548..c5085588e536 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -117,7 +117,8 @@ static int sysfs_rtnl_lock(struct kobject *kobj, struct attribute *attr,
>          * the rtnl lock.
>          */
>  unbreak:
> -       sysfs_unbreak_active_protection(kn);
> +       if (kn)
> +               sysfs_unbreak_active_protection(kn);
>         dev_put(ndev);
>  
>         return ret;
> -- 
> 2.39.5
> 
>