[PATCH net 3/5] net: tun: keep TUN short linear copy hints safe

The issue:
A TUN skb can be redirected by tc mirred to an Ethernet xmit path, which
calls eth_type_trans() and needs a full Ethernet header in the linear
area.  tun_get_user() currently validates len and the GSO header length
only for IFF_TAP, so an IFF_TUN | IFF_VNET_HDR packet can set hdr_len to
1..13 and leave fewer than ETH_HLEN linear bytes.  The redirected packet
then BUGs in eth_type_trans().

The Fix:
Reject packets shorter than ETH_HLEN in both modes.  For IFF_TUN, where
hdr_len is only an allocation/copy hint when GSO and checksum offload are
off, raise a short nonzero hdr_len to ETH_HLEN instead of rejecting the
write.  Keep the TAP rejection and its NET_IP_ALIGN adjustment unchanged.
This preserves valid no-offload TUN writes with hints below ETH_HLEN while
guaranteeing at least ETH_HLEN linear bytes for the redirected consumer.

The maximum receive headroom advertised through tun_set_headroom() must
reserve the same linear minimum.  A large upper-device needed_headroom
(for example an L2 netkit port behind a VLAN in an OVS datapath) otherwise
caps good_linear below ETH_HLEN and defeats the clamp.  Reserve ETH_HLEN
for TUN; the TAP branch already reserves ETH_HLEN + NET_IP_ALIGN.

Conditions to recreate the bug (repro available):
- create a TUN device with IFF_TUN | IFF_VNET_HDR | IFF_NO_PI;
- attach ingress mirred egress redirect dev lo;
- write a virtio_net_hdr with hdr_len=1 and an 8192-byte payload.
Unfixed kernel will panic in eth_type_trans via loopback_xmit.
In a fixed kernel the write is accepted with hdr_len clamped to ETH_HLEN and the
packet is forwarded without a BUG.
- no-offload acceptance: a separate check writes a 28-byte IPv4/UDP-shaped
  packet with hdr_len 0, 1, 13, 14 and 20; all writes are accepted.
- high headroom: create an L2 netkit pair with headroom 4096, put a VLAN
  over it, and add both the VLAN and the TUN to one OVS datapath so the
  TUN receive headroom is raised to 4100.  The same hdr_len=1 jumbo must
  keep at least ETH_HLEN linear bytes; without the setter reservation it
  still BUGs.

Fixes: f42157cb568c ("tun: fallback if skb_alloc() fails on big packets")
Cc: stable@vger.kernel.org
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 drivers/net/tun.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 242899f7fd07..1e235c2f0aee 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -1193,7 +1193,7 @@ static void tun_set_headroom(struct net_device *dev, int new_hr)
 	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP)
 		max_headroom -= ETH_HLEN + NET_IP_ALIGN;
 	else
-		max_headroom -= 1;
+		max_headroom -= ETH_HLEN;
 
 	tun->align = clamp_t(int, new_hr, NET_SKB_PAD, max_headroom);
 }

@@ -1830,6 +1830,11 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		align += NET_IP_ALIGN;
 		if (unlikely(len < ETH_HLEN || (hdr_len && hdr_len < ETH_HLEN)))
 			return -EINVAL;
+	} else {
+		if (unlikely(len < ETH_HLEN))
+			return -EINVAL;
+		if (hdr_len)
+			hdr_len = max_t(int, hdr_len, ETH_HLEN);
 	}
 
 	good_linear = SKB_MAX_HEAD(align);

-- 
2.43.0