[PATCH net] vxlan: use one headroom snapshot for neighbour replies

[Sanghyun Park <sanghyun.park.cnu@gmail.com>]

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 out
of bounds.

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 <sanghyun.park.cnu@gmail.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 459f19f7071e..e5a92d30113c 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -1947,13 +1947,15 @@ static struct sk_buff *vxlan_na_create(struct sk_buff *request,
 	struct ipv6hdr *pip6;
 	u8 *daddr;
 	int na_olen = 8; /* opt hdr + ETH_ALEN for target */
+	int headroom;
 	int ns_olen;
 	int i, len;
 
 	if (dev == NULL || !pskb_may_pull(request, request->len))
 		return NULL;
 
-	len = LL_RESERVED_SPACE(dev) + sizeof(struct ipv6hdr) +
+	headroom = LL_RESERVED_SPACE(dev);
+	len = headroom + sizeof(struct ipv6hdr) +
 		sizeof(*na) + na_olen + dev->needed_tailroom;
 	reply = alloc_skb(len, GFP_ATOMIC);
 	if (reply == NULL)
@@ -1961,7 +1963,7 @@ static struct sk_buff *vxlan_na_create(struct sk_buff *request,
 
 	reply->protocol = htons(ETH_P_IPV6);
 	reply->dev = dev;
-	skb_reserve(reply, LL_RESERVED_SPACE(request->dev));
+	skb_reserve(reply, headroom);
 	skb_push(reply, sizeof(struct ethhdr));
 	skb_reset_mac_header(reply);
 
-- 
2.48.1