Re: [PATCH ipsec] esp: fix page frag reference leak on skb_to_sgvec failure

[Steffen Klassert <steffen.klassert@secunet.com>]

On Mon, Jun 01, 2026 at 03:45:25PM +0200, Alessandro Schino wrote:
> In esp_output_tail(), when esp->inplace is false, the old skb page frags
> are replaced with a new page from the xfrm page_frag cache. The source
> scatterlist (sg) is built from the old frags before the replacement, and
> esp_ssg_unref() is responsible for releasing the old page references
> after the crypto operation completes.
> 
> However, if the second skb_to_sgvec() call (which builds the destination
> scatterlist from the new page) fails, the code jumps to error_free which
> only calls kfree(tmp). The old page frag references captured in the
> source scatterlist are never released:
> 
>   1. sg[] is built from old frags via skb_to_sgvec() (no extra get_page)
>   2. nr_frags is set to 1 and frag[0] is replaced with the new page
>   3. Second skb_to_sgvec() fails -> goto error_free
> 
> Fix this by adding a bool parameter to esp_ssg_unref() that, when true,
> unconditionally unrefs the source scatterlist frags. Since req->src is
> not yet initialized by aead_request_set_crypt() at the point of the
> error, the source scatterlist is obtained directly via esp_req_sg().
> Existing callers pass false to preserve the original behavior.
> 
> The same issue exists in both esp4 and esp6 as the code is identical.
> 
> Fixes: cac2661c53f3 ("esp4: Avoid skb_cow_data whenever possible")
> Fixes: 03e2a30f6a27 ("esp6: Avoid skb_cow_data whenever possible")
> Signed-off-by: Alessandro Schino <7991aleschino@gmail.com>
> ---
>  net/ipv4/esp4.c | 14 ++++++++------
>  net/ipv6/esp6.c | 14 ++++++++------
>  2 files changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
> index 513c8215c947..2429c7845984 100644
> --- a/net/ipv4/esp4.c
> +++ b/net/ipv4/esp4.c
> @@ -96,7 +96,7 @@ static inline struct scatterlist *esp_req_sg(struct crypto_aead *aead,
>  			     __alignof__(struct scatterlist));
>  }
>  
> -static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb)
> +static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb, bool already_unref)
>  {
>  	struct crypto_aead *aead = x->data;
>  	int extralen = 0;
> @@ -113,8 +113,8 @@ static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb)
>  	/* Unref skb_frag_pages in the src scatterlist if necessary.
>  	 * Skip the first sg which comes from skb->data.
>  	 */
> -	if (req->src != req->dst)
> -		for (sg = sg_next(req->src); sg; sg = sg_next(sg))
> +	if (already_unref || req->src != req->dst)
> +		for (sg = sg_next(already_unref ? esp_req_sg(aead, req) : req->src); sg; sg = sg_next(sg))
>  			skb_page_unref(page_to_netmem(sg_page(sg)),
>  				       skb->pp_recycle);
>  }
> @@ -220,7 +220,7 @@ static void esp_output_done(void *data, int err)
>  	}
>  
>  	tmp = ESP_SKB_CB(skb)->tmp;
> -	esp_ssg_unref(x, tmp, skb);
> +	esp_ssg_unref(x, tmp, skb, false);
>  	kfree(tmp);
>  
>  	if (xo && (xo->flags & XFRM_DEV_RESUME)) {
> @@ -569,8 +569,10 @@ int esp_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info *
>  		err = skb_to_sgvec(skb, dsg,
>  			           (unsigned char *)esph - skb->data,
>  			           assoclen + ivlen + esp->clen + alen);
> -		if (unlikely(err < 0))
> +		if (unlikely(err < 0)) {
> +			esp_ssg_unref(x, tmp, skb, true);
>  			goto error_free;
> +		}
>  	}
>  
>  	if ((x->props.flags & XFRM_STATE_ESN))
> @@ -602,7 +604,7 @@ int esp_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info *
>  	}
>  
>  	if (sg != dsg)
> -		esp_ssg_unref(x, tmp, skb);
> +		esp_ssg_unref(x, tmp, skb, false);
>  
>  	if (!err && x->encap && x->encap->encap_type == TCP_ENCAP_ESPINTCP)
>  		err = esp_output_tail_tcp(x, skb);
> diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
> index 57481e423e59..50af6ab9b8fc 100644
> --- a/net/ipv6/esp6.c
> +++ b/net/ipv6/esp6.c
> @@ -113,7 +113,7 @@ static inline struct scatterlist *esp_req_sg(struct crypto_aead *aead,
>  			     __alignof__(struct scatterlist));
>  }
>  
> -static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb)
> +static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb, bool already_unref)
>  {
>  	struct crypto_aead *aead = x->data;
>  	int extralen = 0;
> @@ -130,8 +130,8 @@ static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb)
>  	/* Unref skb_frag_pages in the src scatterlist if necessary.
>  	 * Skip the first sg which comes from skb->data.
>  	 */
> -	if (req->src != req->dst)
> -		for (sg = sg_next(req->src); sg; sg = sg_next(sg))
> +	if (already_unref || req->src != req->dst)
> +		for (sg = sg_next(already_unref ? esp_req_sg(aead, req) : req->src); sg; sg = sg_next(sg))
>  			skb_page_unref(page_to_netmem(sg_page(sg)),
>  				       skb->pp_recycle);
>  }

Now checkpatch complains about this:

WARNING: line length of 106 exceeds 100 columns
#21: FILE: net/ipv4/esp4.c:117:
+               for (sg = sg_next(already_unref ? esp_req_sg(aead, req) : req->src); sg; sg = sg_next(sg))

WARNING: line length of 106 exceeds 100 columns
#75: FILE: net/ipv6/esp6.c:134:
+               for (sg = sg_next(already_unref ? esp_req_sg(aead, req) : req->src); sg; sg = sg_next(sg))


This is not a big issue, but it makes the code unreadable
over time.

Please check your patches before you send it.