Subject: Re: [PATCH 08/17] net: convert sk_filter.refcnt from atomic_t to refcount_t

From: Daniel Borkmann <[email-masked]>
Date: Thu, 16 Mar 2017 17:04:49 +0100

On 03/16/2017 04:28 PM, Elena Reshetova wrote:
> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.
>
> Signed-off-by: Elena Reshetova <[email-masked]>
> Signed-off-by: Hans Liljestrand <[email-masked]>
> Signed-off-by: Kees Cook <[email-masked]>
> Signed-off-by: David Windsor <[email-masked]>
> ---
>   include/linux/filter.h | 3 ++-
>   net/core/filter.c      | 7 ++++---
>   2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 8053c38..20247e7 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -7,6 +7,7 @@
>   #include <stdarg.h>
>
>   #include <linux/atomic.h>
> +#include <linux/refcount.h>
>   #include <linux/compat.h>
>   #include <linux/skbuff.h>
>   #include <linux/linkage.h>
> @@ -431,7 +432,7 @@ struct bpf_prog {
>   };
>
>   struct sk_filter {
> -	atomic_t	refcnt;
> +	refcount_t	refcnt;
>   	struct rcu_head	rcu;
>   	struct bpf_prog	*prog;
>   };
> diff --git a/net/core/filter.c b/net/core/filter.c
> index ebaeaf2..62267e2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -928,7 +928,7 @@ static void sk_filter_release_rcu(struct rcu_head *rcu)
>    */
>   static void sk_filter_release(struct sk_filter *fp)
>   {
> -	if (atomic_dec_and_test(&fp->refcnt))
> +	if (refcount_dec_and_test(&fp->refcnt))
>   		call_rcu(&fp->rcu, sk_filter_release_rcu);
>   }
>
> @@ -950,7 +950,7 @@ bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
>   	/* same check as in sock_kmalloc() */
>   	if (filter_size <= sysctl_optmem_max &&
>   	    atomic_read(&sk->sk_omem_alloc) + filter_size < sysctl_optmem_max) {
> -		atomic_inc(&fp->refcnt);
> +		refcount_inc(&fp->refcnt);
>   		atomic_add(filter_size, &sk->sk_omem_alloc);
>   		return true;
>   	}
> @@ -1179,12 +1179,13 @@ static int __sk_attach_prog(struct bpf_prog *prog, struct sock *sk)
>   		return -ENOMEM;
>
>   	fp->prog = prog;
> -	atomic_set(&fp->refcnt, 0);
> +	refcount_set(&fp->refcnt, 1);
>
>   	if (!sk_filter_charge(sk, fp)) {
>   		kfree(fp);
>   		return -ENOMEM;
>   	}
> +	refcount_set(&fp->refcnt, 1);

Regarding the two subsequent refcount_set(, 1) that look a bit strange
due to the sk_filter_charge() having refcount_inc() I presume ... can't
the refcount API handle such corner case? Or alternatively the let the
sk_filter_charge() handle it, for example:

bool __sk_filter_charge(struct sock *sk, struct sk_filter *fp)
{
	u32 filter_size = bpf_prog_size(fp->prog->len);

	/* same check as in sock_kmalloc() */
	if (filter_size <= sysctl_optmem_max &&
	    atomic_read(&sk->sk_omem_alloc) + filter_size < sysctl_optmem_max) {
		atomic_add(filter_size, &sk->sk_omem_alloc);
		return true;
	}
	return false;
}

And this goes to filter.h:

bool __sk_filter_charge(struct sock *sk, struct sk_filter *fp);

bool sk_filter_charge(struct sock *sk, struct sk_filter *fp)
{
	bool ret = __sk_filter_charge(sk, fp);
	if (ret)
		refcount_inc(&fp->refcnt);
	return ret;
}

... and let __sk_attach_prog() call __sk_filter_charge() and only fo
the second refcount_set()?

>   	old_fp = rcu_dereference_protected(sk->sk_filter,
>   					   lockdep_sock_is_held(sk));
>