FAILED: patch "[PATCH] fou: Fix use-after-free in fou_create()" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x b14361aca6350ff7907b0e9903c7b94dc7d5d4a0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026080557-pony-parameter-ab69@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b14361aca6350ff7907b0e9903c7b94dc7d5d4a0 Mon Sep 17 00:00:00 2001
From: Xuanqiang Luo <[email protected]>
Date: Wed, 22 Jul 2026 16:38:58 +0800
Subject: [PATCH] fou: Fix use-after-free in fou_create()

fou_create() publishes struct fou through sk_user_data before adding the
new FOU port to the per-netns list.  If fou_add_to_port_list() fails,
the error path frees fou while it is still reachable through
sk_user_data.  A concurrent receive can then dereference the freed
object in fou_from_sock().

This ordering issue was previously noted in the linked discussion.

The failure is reachable when local port 0 is requested.  Each socket
binds to a different ephemeral port, but fou_cfg_cmp() compares the
requested port 0 and reports -EALREADY once an entry already exists.

Release the tunnel socket before freeing fou so sk_user_data is cleared
first, and defer reclamation with kfree_rcu() to protect concurrent RCU
readers.  This matches the lifetime handling in fou_release().

Fixes: 23461551c006 ("fou: Support for foo-over-udp RX path")
Suggested-by: Kuniyuki Iwashima <[email protected]>
Link: https://lore.kernel.org/netdev/[email protected]/
Cc: [email protected]
Signed-off-by: Xuanqiang Luo <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

diff --git a/net/ipv4/fou_core.c b/net/ipv4/fou_core.c
index 865bd7205122..ab09dfcdecbd 100644
--- a/net/ipv4/fou_core.c
+++ b/net/ipv4/fou_core.c
@@ -629,9 +629,9 @@ static int fou_create(struct net *net, struct fou_cfg *cfg,
 	return 0;
 
 error:
-	kfree(fou);
 	if (sock)
 		udp_tunnel_sock_release(sock->sk);
+	kfree_rcu(fou, rcu);
 
 	return err;
 }