[PATCH net] geneve: ensure the skb is writable before fixing its headers

[Antoine Tenart <atenart@kernel.org>]

Make sure the IPv4/6 and UDP headers are writable before fixing them up in
geneve_post_decap_hint. As skb_ensure_writable can reallocate the skb linear
area, reload the GRO hint header pointer and only set the IPv4/6 header ones
after the call.

Fixes: fd0dd796576e ("geneve: use GRO hint option in the RX path")
Cc: Paolo Abeni <pabeni@redhat.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260529144713.780938-1-atenart%40kernel.org
Signed-off-by: Antoine Tenart <atenart@kernel.org>
---
 drivers/net/geneve.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/net/geneve.c b/drivers/net/geneve.c
index b25b0c4de4d1..0b0f1e300b68 100644
--- a/drivers/net/geneve.c
+++ b/drivers/net/geneve.c
@@ -586,6 +586,7 @@ static int geneve_post_decap_hint(const struct sock *sk, struct sk_buff *skb,
 	struct iphdr *iph;
 	struct udphdr *uh;
 	__be16 p;
+	int err;
 
 	hint_off = geneve_sk_gro_hint_off(sk, *geneveh, &p, &len);
 	if (!hint_off)
@@ -610,12 +611,20 @@ static int geneve_post_decap_hint(const struct sock *sk, struct sk_buff *skb,
 		     !geneve_opt_gro_hint_validate(skb->data, gro_hint)))
 		return -EINVAL;
 
-	ipv6h = (void *)skb->data + gro_hint->nested_nh_offset;
-	iph = (struct iphdr *)ipv6h;
 	total_len = skb->len - gro_hint->nested_nh_offset;
 	if (total_len >= GRO_LEGACY_MAX_SIZE)
 		return -E2BIG;
 
+	err = skb_ensure_writable(skb, gro_hint->nested_tp_offset + sizeof(*uh));
+	if (unlikely(err))
+		return err;
+
+	*geneveh = geneve_hdr(skb);
+	gro_hint = geneve_opt_gro_hint(*geneveh, hint_off);
+
+	ipv6h = (void *)skb->data + gro_hint->nested_nh_offset;
+	iph = (struct iphdr *)ipv6h;
+
 	/*
 	 * After stripping the outer encap, the packet still carries a
 	 * tunnel encapsulation: the nested one.
-- 
2.55.0