[PATCH net] tun: prevent underflow in rx headroom calculation

[Asim Viladi Oglu Manizada <manizada@pm.me>]

tun_get_user() calculates good_linear with SKB_MAX_HEAD(align). The align
value comes from ndo_set_rx_headroom() and can be larger than the linear
space available in a one-page skb head.

OVS can reach this case by carrying headroom from a netkit/VXLAN port to a
TUN port. SKB_MAX_HEAD() then underflows, leaving good_linear negative.
Assigning that value to the size_t linear variable in tun_get_user()
converts it to a large positive value. The wrapped value is passed to
tun_alloc_skb(), where prepad + linear and len - linear wrap. skb->data can
then end up past the allocated head, and later packet processing can access
memory outside the skb.

Clamp good_linear to zero when SKB_MAX_HEAD() returns a negative value.
This lets tun_alloc_skb() allocate the requested headroom and place the
packet data linearly or in fragments without wrapping.

Fixes: eaea34b23c46 ("net/tun: implement ndo_set_rx_headroom")
Cc: stable@vger.kernel.org
Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
---
 drivers/net/tun.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index ffbe6f13fb1..d3be0f2d5df 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1780,6 +1780,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	}
 
 	good_linear = SKB_MAX_HEAD(align);
+	good_linear = max(good_linear, 0);
 
 	if (msg_control) {
 		struct iov_iter i = *from;
-- 
2.39.5