linux-kernel - Re: [PATCH] macsec: dynamically allocate space for sglist

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Message-ID: <20170425163602.GA17973@bistromath.localdomain>
Date:   Tue, 25 Apr 2017 18:36:02 +0200
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>, stable@...r.kernel.org,
        security@...nel.org
Subject: Re: [PATCH] macsec: dynamically allocate space for sglist

2017-04-25, 17:23:00 +0200, Jason A. Donenfeld wrote:
> We call skb_cow_data, which is good anyway to ensure we can actually
> modify the skb as such (another error from prior). Now that we have the
> number of fragments required, we can safely allocate exactly that amount
> of memory.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> Cc: Sabrina Dubroca <sd@...asysnail.net>
> Cc: security@...nel.org
> Cc: stable@...r.kernel.org
> ---
>  drivers/net/macsec.c | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
> index dbab05afcdbe..56dafdee4c9c 100644
> --- a/drivers/net/macsec.c
> +++ b/drivers/net/macsec.c
[...]
> @@ -917,6 +926,7 @@ static struct sk_buff *macsec_decrypt(struct sk_buff *skb,
>  {
>  	int ret;
>  	struct scatterlist *sg;
> +	struct sk_buff *trailer;
>  	unsigned char *iv;
>  	struct aead_request *req;
>  	struct macsec_eth_header *hdr;
> @@ -927,7 +937,12 @@ static struct sk_buff *macsec_decrypt(struct sk_buff *skb,
>  	if (!skb)
>  		return ERR_PTR(-ENOMEM);
>  
> -	req = macsec_alloc_req(rx_sa->key.tfm, &iv, &sg);
> +	ret = skb_cow_data(skb, 0, &trailer);
> +	if (unlikely(ret < 0)) {
> +		kfree_skb(skb);
> +		return ERR_PTR(ret);
> +	}
> +	req = macsec_alloc_req(rx_sa->key.tfm, &iv, &sg, ret);
>  	if (!req) {
>  		kfree_skb(skb);
>  		return ERR_PTR(-ENOMEM);

There's a problem here (and in macsec_encrypt): you need to update the
call to sg_init_table, like I did in my patch.  Otherwise,
sg_init_table() is going to access sg[MAX_SKB_FRAGS], which may be
past what you allocated.

How did you test this? ;)

-- 
Sabrina