[PATCH net v2] esp: downgrade zerocopy managed frags before mutating skb frags

[Maher Azzouzi <maherazz04@gmail.com>]

On the out-of-place output path (esp->inplace == false) ESP rewrites the
skb frag array: esp_output_head() appends a trailer frag and
esp_output_tail() replaces the frags with a destination page, both
referenced with get_page().

When the skb carries zerocopy managed frags (SKBFL_MANAGED_FRAG_REFS) the
payload frags are owned by the ubuf and must not be referenced or
unreferenced individually, but ESP mutates the frag array without ever
downgrading the skb.  This breaks the managed-frag invariant two ways:

  - esp_ssg_unref() walks the source scatterlist and drops a page
    reference for every frag, including the ubuf-owned payload frags,
    pushing their refcount below the GUP pin bias while the pages are
    still pinned, i.e. a use-after-free of the zerocopy pages;

  - esp_output_tail() installs its destination page as frag 0 with
    get_page() but leaves SKBFL_MANAGED_FRAG_REFS set, so
    skb_release_data() takes the skip_unref branch and never drops that
    reference, leaking the x->xfrag page at packet rate.

Fix this the way every other frag-mutating site does (__ip_append_data(),
__ip6_append_data(), tcp_sendmsg_locked()) and call
skb_zcopy_downgrade_managed() before ESP touches the frag array: it takes
a real reference on each existing frag and clears SKBFL_MANAGED_FRAG_REFS,
so the per-frag unref in esp_ssg_unref() and the frag release in
skb_release_data() are both balanced and no mixed-ownership frag array is
left behind.

Fixes: 753f1ca4e1e5 ("net: introduce managed frags infrastructure")
Signed-off-by: Maher Azzouzi <maherazz04@gmail.com>
---
Changes in v2:
 - Use skb_zcopy_downgrade_managed() before ESP mutates the frag array,
   instead of the early return in esp_ssg_unref().  As pointed out in
   review, that early return also suppressed release of the ESP-owned
   trailer and destination page references (a leak); downgrading the skb
   the way __ip_append_data() does fixes both the underflow and the leak.
 - Fix the Fixes: tag - the bug needs managed frags (753f1ca4e1e5), not
   the 2017 skb_cow_data avoidance commits.

v1: https://lore.kernel.org/netdev/20260712170530.9807-1-maherazz04@gmail.com/

 net/ipv4/esp4.c | 6 ++++++
 net/ipv6/esp6.c | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index dfc81ee..faa48f5 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -441,6 +441,12 @@ int esp_output_head(struct xfrm_state *x, struct sk_buff *skb, struct esp_info *
 
 			esp->inplace = false;
 
+			/* Take real page refs and clear SKBFL_MANAGED_FRAG_REFS before
+			 * we mutate the frag array, so the per-frag unref stays balanced
+			 * for zerocopy managed frags (see __ip_append_data()).
+			 */
+			skb_zcopy_downgrade_managed(skb);
+
 			allocsize = ALIGN(tailen, L1_CACHE_BYTES);
 
 			spin_lock_bh(&x->lock);
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 296b579..a3a3857 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -470,6 +470,12 @@ int esp6_output_head(struct xfrm_state *x, struct sk_buff *skb, struct esp_info
 
 			esp->inplace = false;
 
+			/* Take real page refs and clear SKBFL_MANAGED_FRAG_REFS before
+			 * we mutate the frag array, so the per-frag unref stays balanced
+			 * for zerocopy managed frags (see __ip_append_data()).
+			 */
+			skb_zcopy_downgrade_managed(skb);
+
 			allocsize = ALIGN(tailen, L1_CACHE_BYTES);
 
 			spin_lock_bh(&x->lock);
-- 
2.34.1