[PATCH 6.1 510/609] udp: fix potential use-after-free in tunnel segmentation

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xuanqiang Luo <[email protected]>

[ Upstream commit d0f86fb36eb260abd10007b62c9dcc1028e03e61 ]

__skb_udp_tunnel_segment() gets the UDP header before ensuring the
tunnel header is in the skb head. If the pull reallocates skb->head,
the saved UDP header pointer is no longer valid.

Get the UDP header after the pull to avoid a potential use-after-free.

Fixes: dbef491ebe7f ("udp: Use uh->len instead of skb->len to compute checksum in segmentation")
Signed-off-by: Xuanqiang Luo <[email protected]>
Reviewed-by: Antoine Tenart <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---
 net/ipv4/udp_offload.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 35c014e10f24b..60fbb4ca34e00 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -21,17 +21,19 @@ static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb,
 	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
 	bool remcsum, need_csum, offload_csum, gso_partial;
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
-	struct udphdr *uh = udp_hdr(skb);
 	u16 mac_offset = skb->mac_header;
 	__be16 protocol = skb->protocol;
 	u16 mac_len = skb->mac_len;
 	int udp_offset, outer_hlen;
+	struct udphdr *uh;
 	__wsum partial;
 	bool need_ipsec;
 
 	if (unlikely(!pskb_may_pull(skb, tnl_hlen)))
 		goto out;
 
+	uh = udp_hdr(skb);
+
 	/* Adjust partial header checksum to negate old length.
 	 * We cannot rely on the value contained in uh->len as it is
 	 * possible that the actual value exceeds the boundaries of the
-- 
2.53.0