[PATCH net] xfrm: account XFRMA_IF_ID in aevent size calculation

[Keenan Dong <keenanat2000@gmail.com>]

xfrm_get_ae() allocates the reply skb with xfrm_aevent_msgsize(), then
build_aevent() appends attributes including XFRMA_IF_ID when x->if_id is
set.

xfrm_aevent_msgsize() does not include space for XFRMA_IF_ID. For states
with if_id, build_aevent() can fail with -EMSGSIZE and hit BUG_ON(err < 0)
in xfrm_get_ae(), turning a malformed netlink interaction into a kernel
panic.

Account XFRMA_IF_ID in the size calculation unconditionally and replace
the BUG_ON with normal error unwinding.

Fixes: 7e6526404ade ("xfrm: Add a new lookup key to match xfrm interfaces.")
Reported-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Keenan Dong <keenanat2000@gmail.com>
---
 net/xfrm/xfrm_user.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index 1656b487f..d79240a1c 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -2677,7 +2677,8 @@ static inline unsigned int xfrm_aevent_msgsize(struct xfrm_state *x)
 	       + nla_total_size(4) /* XFRM_AE_RTHR */
 	       + nla_total_size(4) /* XFRM_AE_ETHR */
 	       + nla_total_size(sizeof(x->dir)) /* XFRMA_SA_DIR */
-	       + nla_total_size(4); /* XFRMA_SA_PCPU */
+	       + nla_total_size(4) /* XFRMA_SA_PCPU */
+	       + nla_total_size(sizeof(x->if_id)); /* XFRMA_IF_ID */
 }
 
 static int build_aevent(struct sk_buff *skb, struct xfrm_state *x, const struct km_event *c)
@@ -2789,7 +2790,12 @@ static int xfrm_get_ae(struct sk_buff *skb, struct nlmsghdr *nlh,
 	c.portid = nlh->nlmsg_pid;
 
 	err = build_aevent(r_skb, x, &c);
-	BUG_ON(err < 0);
+	if (err < 0) {
+		spin_unlock_bh(&x->lock);
+		xfrm_state_put(x);
+		kfree_skb(r_skb);
+		return err;
+	}
 
 	err = nlmsg_unicast(xfrm_net_nlsk(net, skb), r_skb, NETLINK_CB(skb).portid);
 	spin_unlock_bh(&x->lock);
-- 
2.43.0