[PATCH net v3] net: tun: bound receive headroom

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

tun_get_user() uses tun->align both as skb headroom and when choosing how
much packet data to keep linear. OVS can propagate an oversized headroom
request from another port to TUN or TAP.

When align is larger than the usable space in a one-page skb head,
SKB_MAX_HEAD(align) underflows and the result becomes negative when stored
in good_linear. That value later wraps when assigned to the size_t linear
variable, and tun_alloc_skb() can place skb->data outside the allocated
head.

Bound the headroom stored by TUN to the one-page skb-head budget and the
largest non-sentinel 16-bit skb header offset. Leave one linear byte for
raw TUN and a complete Ethernet header for TAP, including NET_IP_ALIGN.

Also pull the raw-TUN protocol byte and the TAP Ethernet header before
accessing them, so these checks remain safe for nonlinear skbs supplied by
other allocation paths.

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>
---
v3:
- simplify the raw-TUN pull check and reject short packets with -EINVAL
v2: https://lore.kernel.org/netdev/20260805084504.953162-1-manizada@pm.me/
- bound tun->align instead of clamping good_linear to zero
- derive the bound from the one-page head, 16-bit offset, and TUN/TAP
  linear-header requirements
- pull the raw-TUN protocol byte before reading it
- make the TAP Ethernet-header pull unconditional
v1: https://lore.kernel.org/netdev/20260721014117.2234892-1-manizada@pm.me/

 drivers/net/tun.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index fed9dfdfcc3b..5bbe3123979e 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1107,11 +1107,16 @@ static netdev_features_t tun_net_fix_features(struct net_device *dev,
 static void tun_set_headroom(struct net_device *dev, int new_hr)
 {
 	struct tun_struct *tun = netdev_priv(dev);
+	size_t max_headroom;
 
-	if (new_hr < NET_SKB_PAD)
-		new_hr = NET_SKB_PAD;
+	max_headroom = min_t(size_t, SKB_MAX_HEAD(0), U16_MAX - 1);
 
-	tun->align = new_hr;
+	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP)
+		max_headroom -= ETH_HLEN + NET_IP_ALIGN;
+	else
+		max_headroom -= 1;
+
+	tun->align = clamp_t(int, new_hr, NET_SKB_PAD, max_headroom);
 }
 
 static void
@@ -1822,7 +1827,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	switch (tun->flags & TUN_TYPE_MASK) {
 	case IFF_TUN:
 		if (tun->flags & IFF_NO_PI) {
-			u8 ip_version = skb->len ? (skb->data[0] >> 4) : 0;
+			u8 ip_version;
+
+			if (!pskb_may_pull(skb, 1)) {
+				err = -EINVAL;
+				goto drop;
+			}
+			ip_version = skb->data[0] >> 4;
 
 			switch (ip_version) {
 			case 4:
@@ -1842,7 +1853,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		skb->dev = tun->dev;
 		break;
 	case IFF_TAP:
-		if (frags && !pskb_may_pull(skb, ETH_HLEN)) {
+		if (!pskb_may_pull(skb, ETH_HLEN)) {
 			err = -ENOMEM;
 			drop_reason = SKB_DROP_REASON_HDR_TRUNC;
 			goto drop;
-- 
2.53.0