linux-kernel - Re: [PATCH] net: convert sk_filter.refcnt from atomic_t to refcount

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Message-ID: <a19263a2-141b-22ad-4300-0ffe54cae8d6@cogentembedded.com>
Date:   Sat, 18 Mar 2017 17:33:22 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Elena Reshetova <elena.reshetova@...el.com>, daniel@...earbox.net
Cc:     ast@...com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, keescook@...omium.org,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH] net: convert sk_filter.refcnt from atomic_t to refcount_t

Hello!

On 3/18/2017 3:58 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 <elena.reshetova@...el.com>
> Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: David Windsor <dwindsor@...il.com>
[...]
> 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
[...]
> @@ -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);

    Why do it twice?

[...]

MBR, Sergei