Re: [RFC 04/10] kmod: provide wrappers for kmod_concurrent inc/dec

[Kees Cook]

On Thu, Dec 8, 2016 at 11:48 AM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> kmod_concurrent is used as an atomic counter for enabling
> the allowed limit of modprobe calls, provide wrappers for it
> to enable this to be expanded on more easily. This will be done
> later.
>
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> ---
>  kernel/kmod.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index cb6f7ca7b8a5..049d7eabda38 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -44,6 +44,9 @@
>  #include <trace/events/module.h>
>
>  extern int max_threads;
> +
> +static atomic_t kmod_concurrent = ATOMIC_INIT(0);
> +
>  unsigned int max_modprobes;
>  module_param(max_modprobes, uint, 0644);
>  MODULE_PARM_DESC(max_modprobes, "Max number of allowed concurrent modprobes");
> @@ -108,6 +111,20 @@ static int call_modprobe(char *module_name, int wait)
>         return -ENOMEM;
>  }
>
> +static int kmod_umh_threads_get(void)
> +{
> +       atomic_inc(&kmod_concurrent);
> +       if (atomic_read(&kmod_concurrent) < max_modprobes)
> +               return 0;
> +       atomic_dec(&kmod_concurrent);
> +       return -ENOMEM;
> +}
> +
> +static void kmod_umh_threads_put(void)
> +{
> +       atomic_dec(&kmod_concurrent);
> +}

Can you use a kref here instead? We're trying to kill raw use of
atomic_t for reference counting...

> +
>  /**
>   * __request_module - try to load a kernel module
>   * @wait: wait (or not) for the operation to complete
> @@ -129,7 +146,6 @@ int __request_module(bool wait, const char *fmt, ...)
>         va_list args;
>         char module_name[MODULE_NAME_LEN];
>         int ret;
> -       static atomic_t kmod_concurrent = ATOMIC_INIT(0);
>         static int kmod_loop_msg;
>
>         /*
> @@ -153,8 +169,8 @@ int __request_module(bool wait, const char *fmt, ...)
>         if (ret)
>                 return ret;
>
> -       atomic_inc(&kmod_concurrent);
> -       if (atomic_read(&kmod_concurrent) > max_modprobes) {
> +       ret = kmod_umh_threads_get();
> +       if (ret) {
>                 /* We may be blaming an innocent here, but unlikely */
>                 if (kmod_loop_msg < 5) {
>                         printk(KERN_ERR
> @@ -162,15 +178,14 @@ int __request_module(bool wait, const char *fmt, ...)
>                                module_name);
>                         kmod_loop_msg++;
>                 }
> -               atomic_dec(&kmod_concurrent);
> -               return -ENOMEM;
> +               return ret;
>         }
>
>         trace_module_request(module_name, wait, _RET_IP_);
>
>         ret = call_modprobe(module_name, wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);
>
> -       atomic_dec(&kmod_concurrent);
> +       kmod_umh_threads_put();
>         return ret;
>  }
>  EXPORT_SYMBOL(__request_module);
> --
> 2.10.1
>



-- 
Kees Cook
Nexus Security