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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , David Stevens , linux-kernel@vger.kernel.org Subject: [PATCH net v2] vxlan: use one headroom snapshot for neighbour replies Date: Fri, 18 Sep 2026 12:26:58 +0900 Message-ID: <20260918032842.502409-2-sanghyun.park.cnu@gmail.com> X-Mailer: git-send-email 2.48.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" vxlan_na_create() samples LL_RESERVED_SPACE() to size the reply skb and then samples it again to reserve headroom. A concurrent vxlan_changelink() can update needed_headroom between the two reads, creating a TOCTOU race. The second value can exceed the allocation and make the Ethernet header write o= ut of bounds. The race is reproducible on the unpatched kernel. It occurred when vxlan_na_create() generated a neighbour reply while vxlan_changelink() chan= ged the link headroom. KASAN caught a four-byte write two bytes beyond a 704-by= te skbuff_small_head allocation. Snapshot the headroom once and use that value for both allocation and reservation. Fixes: 4b29dba9c085 ("vxlan: fix nonfunctional neigh_reduce()") Signed-off-by: Sanghyun Park --- v2: - Improve the v1 changelog v1: https://lore.kernel.org/r/20260831054614.2069896-2-sanghyun.park.cnu@gm= ail.com drivers/net/vxlan/vxlan_core.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c index c1d54339fa2b..3390e341d1e5 100644 --- a/drivers/net/vxlan/vxlan_core.c +++ b/drivers/net/vxlan/vxlan_core.c @@ -1958,13 +1958,15 @@ static struct sk_buff *vxlan_na_create(struct sk_bu= ff *request, struct ipv6hdr *pip6; u8 *daddr; int na_olen =3D 8; /* opt hdr + ETH_ALEN for target */ + int headroom; int ns_olen; int i, len; =20 if (dev =3D=3D NULL || !pskb_may_pull(request, request->len)) return NULL; =20 - len =3D LL_RESERVED_SPACE(dev) + sizeof(struct ipv6hdr) + + headroom =3D LL_RESERVED_SPACE(dev); + len =3D headroom + sizeof(struct ipv6hdr) + sizeof(*na) + na_olen + dev->needed_tailroom; reply =3D alloc_skb(len, GFP_ATOMIC); if (reply =3D=3D NULL) @@ -1972,7 +1974,7 @@ static struct sk_buff *vxlan_na_create(struct sk_buff= *request, =20 reply->protocol =3D htons(ETH_P_IPV6); reply->dev =3D dev; - skb_reserve(reply, LL_RESERVED_SPACE(request->dev)); + skb_reserve(reply, headroom); skb_push(reply, sizeof(struct ethhdr)); skb_reset_mac_header(reply); =20 --=20 2.48.1