[PATCH net] esp: do not unref managed frag pages in esp_ssg_unref()

[Maher Azzouzi]

esp_ssg_unref() releases the page references held on the source
scatterlist after the AEAD operation completes.  It calls
skb_page_unref() on every frag page for an out-of-place transform
(req->src != req->dst), and in the error path of esp_output_tail()
(already_unref == true) on the request's own scatterlist.

This is wrong when the skb carries managed frags
(SKBFL_MANAGED_FRAG_REFS).  Managed frags are owned by a zerocopy ubuf
and the skb does not hold a per-frag page reference; io_uring SEND_ZC
with a registered buffer attaches the bvec pages this way via
io_sg_from_iter().  The rest of the stack honours this invariant:
skb_release_data() skips the per-frag unref when SKBFL_MANAGED_FRAG_REFS
is set, and skb_zcopy_managed() is the guard used at the other unref
sites.

esp_ssg_unref() is missing that guard, so for a managed-frag skb it
drops a page reference the skb never acquired.  This can underflow the
page reference count and free a page that is still in use.

Guard the function with skb_zcopy_managed() so both unref paths are
skipped for managed-frag skbs, matching skb_release_data().

Fixes: cac2661c53f3 ("esp4: Avoid skb_cow_data whenever possible")
Fixes: 03e2a30f6a27 ("esp6: Avoid skb_cow_data whenever possible")
Signed-off-by: Maher Azzouzi <maherazz04@xxxxxxxxx>
---
 net/ipv4/esp4.c | 7 +++++++
 net/ipv6/esp6.c | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index dfc81ee..fa1710e 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -104,6 +104,13 @@ static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb,
 	struct aead_request *req;
 	struct scatterlist *sg;
 
+	/* Managed frags are owned by the zerocopy ubuf; the skb holds no
+	 * per-frag page reference, so we must not drop one here.  Mirrors
+	 * the SKBFL_MANAGED_FRAG_REFS handling in skb_release_data().
+	 */
+	if (skb_zcopy_managed(skb))
+		return;
+
 	if (x->props.flags & XFRM_STATE_ESN)
 		extralen += sizeof(struct esp_output_extra);
 
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 296b579..7d216b9 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -121,6 +121,13 @@ static void esp_ssg_unref(struct xfrm_state *x, void *tmp, struct sk_buff *skb,
 	struct aead_request *req;
 	struct scatterlist *sg;
 
+	/* Managed frags are owned by the zerocopy ubuf; the skb holds no
+	 * per-frag page reference, so we must not drop one here.  Mirrors
+	 * the SKBFL_MANAGED_FRAG_REFS handling in skb_release_data().
+	 */
+	if (skb_zcopy_managed(skb))
+		return;
+
 	if (x->props.flags & XFRM_STATE_ESN)
 		extralen += sizeof(struct esp_output_extra);
 
-- 
2.34.1