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

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

[Sanghyun Park]

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

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

[Paolo Abeni]

On 8/31/26 7:46 AM, Sanghyun Park wrote:
> 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>
The patch LGTM, but the changelog needs some improvements: if you have
can observe a splat on the unpatched kernel, please include it,
otherwise please explain whythe issue is not just a theoretical one.
Also please specify if some LLM has been used.

Thanks,

Paolo

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

[Sanghyun Park]

Thanks for the review, Paolo.

> if you can observe a splat on the unpatched kernel, please include it,
> otherwise please explain why the issue is not just a theoretical one.

I thought it would make the changelog verbose by putting the evidences of the
bugs (some people like concise changelogs).

If you prefer splats in there, let me add some in v2.

It would be like:

```
/* vxlan_na_create() samples ... */

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 ... */
```

JFYI, every patch I make is done only after I get real evidence :)

> Also please specify if some LLM has been used.

I used LLM assistance during bug discovery, analysis, etc, but try to review
with my eyes as much as I can.


Plz check the new paragraph of the changelog whether it is ok for v2.

Thanks,
Sanghyun