xfrm: fix use-after-free of migrated state in xfrm_do_migrate_state()

A concurrent delete can free the migrated state before
xfrm_do_migrate_state() finishes using it.

Fixes: a9d155ea9b44 ("xfrm: add XFRM_MSG_MIGRATE_STATE for single SA migration")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index 5a9f729..8640838 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -3509,6 +3509,7 @@ static int xfrm_do_migrate_state(struct sk_buff *skb, struct nlmsghdr *nlh,
 	__xfrm_state_delete(x);
 	spin_unlock_bh(&x->lock);
 
+	xfrm_state_hold(xc);
 	err = xfrm_state_migrate_install(x, xc, &m, extack);
 	if (err < 0) {
 		/*
@@ -3516,6 +3517,7 @@ static int xfrm_do_migrate_state(struct sk_buff *skb, struct nlmsghdr *nlh,
 		 * free under xfrm_cfg_mutex. Both SAs are gone if it does;
 		 * restoring x would risk SN/IV reuse.
 		 */
+		xfrm_state_put(xc);
 		goto out;
 	}
 
@@ -3534,6 +3536,7 @@ static int xfrm_do_migrate_state(struct sk_buff *skb, struct nlmsghdr *nlh,
 		err = 0;
 	}
 
+	xfrm_state_put(xc);
 out:
 	xfrm_state_put(x);
 	return err;