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

[Alessandro Schino <7991aleschino@gmail.com>]

From: e521588 <alessandro.schino@sbb.ch>

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
  4. kfree(tmp) frees the sg[] memory but old frags are not unref'd
  5. kfree_skb() only releases frag[0] (the new page), not the old ones

Fix this by calling esp_ssg_unref() before the goto error_free when the
second skb_to_sgvec() fails. At this point, the source scatterlist in
tmp is fully populated, so esp_ssg_unref() can properly release all old
page frag references.

The same issue exists in both esp4 and esp6 as the code is identical.

Signed-off-by: Alessandro Schino <7991aleschino@gmail.com>
---
 net/ipv4/esp4.c | 4 +++-
 net/ipv6/esp6.c | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index 6a5febbdbee4..5c4b162c1bb5 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -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);
 			goto error_free;
+		}
 	}
 
 	if ((x->props.flags & XFRM_STATE_ESN))
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 9c06c5a1419d..9166e1da3615 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -600,8 +600,10 @@ int esp6_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);
 			goto error_free;
+		}
 	}
 
 	if ((x->props.flags & XFRM_STATE_ESN))
-- 
2.41.0.windows.3