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vxlan: use one headroom snapshot for neighbour replies

Browse Source 
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.

The race is reproducible on the unpatched kernel. It occurred when
vxlan_na_create() generated a neighbour reply while vxlan_changelink() changed
the link headroom. KASAN caught a four-byte write two bytes beyond a 704-byte
skbuff_small_head allocation.

Snapshot the headroom once and use that value for both allocation and
reservation.

Fixes: 4b29dba9c0 ("vxlan: fix nonfunctional neigh_reduce()")
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>
Link: https://patch.msgid.link/20260918032842.502409-2-sanghyun.park.cnu@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Sanghyun Park 2026-09-18 12:26:58 +09:00 committed by Jakub Kicinski
parent 0f2fd31f63
commit 481506a756
1 changed files with 4 additions and 2 deletions
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6
drivers/net/vxlan/vxlan_core.c
View File

@ -1958,13 +1958,15 @@ 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)
@ -1972,7 +1974,7 @@ 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);