[PATCH v2] net: bpf: permit redirect from L3 to L2 devices at near max mtu

["Maciej Żenczykowski" <zenczykowski@gmail.com>]

From: Maciej Żenczykowski <maze@google.com>

__bpf_skb_max_len(skb) is used from:
  bpf_skb_adjust_room
  __bpf_skb_change_tail
  __bpf_skb_change_head

but in the case of forwarding we're likely calling these functions
during receive processing on ingress and bpf_redirect()'ing at
a later point in time to egress on another interface, thus these
mtu checks are for the wrong device (input instead of output).

This is particularly problematic if we're receiving on an L3 1500 mtu
cellular interface, trying to add an L2 header and forwarding to
an L3 mtu 1500 mtu wifi/ethernet device (which is thus L2 1514).

The mtu check prevents us from adding the 14 byte ethernet header prior
to forwarding the packet.

After the packet has already been redirected, we'd need to add
an additional 2nd ebpf program on the target device's egress tc hook,
but then we'd also see non-redirected traffic and have no easy
way to tell apart normal egress with ethernet header packets
from forwarded ethernet headerless packets.

Credits to Alexei Starovoitov for the suggestion on how to 'fix' this.

This probably should be further fixed up *somehow*, just
not at all clear how.  This does at least make things work.

Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
---
 net/core/filter.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 7d6ceaa54d21..811aba77e24b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3159,8 +3159,20 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 
 static u32 __bpf_skb_max_len(const struct sk_buff *skb)
 {
-	return skb->dev ? skb->dev->mtu + skb->dev->hard_header_len :
-			  SKB_MAX_ALLOC;
+	if (skb->dev) {
+		unsigned short header_len = skb->dev->hard_header_len;
+
+		/* HACK: Treat 0 as ETH_HLEN to allow redirect while
+		 * adding ethernet header from an L3 (tun, rawip, cellular)
+		 * to an L2 device (tap, ethernet, wifi)
+		 */
+		if (!header_len)
+			header_len = ETH_HLEN;
+
+		return skb->dev->mtu + header_len;
+	} else {
+		return SKB_MAX_ALLOC;
+	}
 }
 
 BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
-- 
2.26.2.526.g744177e7f7-goog