On Sat, 23 Jan 2021 02:35:41 +0100
Daniel Borkmann [off-list ref] wrote:

quoted

+ *		The *flags* argument can be a combination of one or more of the
+ *		following values:
+ *
+ *		**BPF_MTU_CHK_SEGS**
+ *			This flag will only works for *ctx* **struct sk_buff**.
+ *			If packet context contains extra packet segment buffers
+ *			(often knows as GSO skb), then MTU check is harder to
+ *			check at this point, because in transmit path it is
+ *			possible for the skb packet to get re-segmented
+ *			(depending on net device features).  This could still be
+ *			a MTU violation, so this flag enables performing MTU
+ *			check against segments, with a different violation
+ *			return code to tell it apart. Check cannot use len_diff.
+ *
+ *		On return *mtu_len* pointer contains the MTU value of the net
+ *		device.  Remember the net device configured MTU is the L3 size,
+ *		which is returned here and XDP and TX length operate at L2.
+ *		Helper take this into account for you, but remember when using
+ *		MTU value in your BPF-code.  On input *mtu_len* must be a valid
+ *		pointer and be initialized (to zero), else verifier will reject
+ *		BPF program.
+ *
+ *	Return
+ *		* 0 on success, and populate MTU value in *mtu_len* pointer.
+ *
+ *		* < 0 if any input argument is invalid (*mtu_len* not updated)
+ *
+ *		MTU violations return positive values, but also populate MTU
+ *		value in *mtu_len* pointer, as this can be needed for
+ *		implementing PMTU handing:
+ *
+ *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
+ *		* **BPF_MTU_CHK_RET_SEGS_TOOBIG**
+ *
   */  

[...]

quoted

+BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
+	   u32, ifindex, u32 *, mtu_len, s32, len_diff, u64, flags)
+{
+	int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
+	struct net_device *dev = skb->dev;
+	int skb_len, dev_len;
+	int mtu;
+
+	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
+		return -EINVAL;
+
+	dev = __dev_via_ifindex(dev, ifindex);
+	if (unlikely(!dev))
+		return -ENODEV;
+
+	mtu = READ_ONCE(dev->mtu);
+
+	dev_len = mtu + dev->hard_header_len;
+	skb_len = skb->len + len_diff; /* minus result pass check */
+	if (skb_len <= dev_len) {
+		ret = BPF_MTU_CHK_RET_SUCCESS;
+		goto out;
+	}
+	/* At this point, skb->len exceed MTU, but as it include length of all
+	 * segments, it can still be below MTU.  The SKB can possibly get
+	 * re-segmented in transmit path (see validate_xmit_skb).  Thus, user
+	 * must choose if segs are to be MTU checked.
+	 */
+	if (skb_is_gso(skb)) {
+		ret = BPF_MTU_CHK_RET_SUCCESS;
+
+		if (flags & BPF_MTU_CHK_SEGS &&
+		    !skb_gso_validate_network_len(skb, mtu))
+			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;  

I think that looks okay overall now. One thing that will easily slip through
is that in the helper description you mentioned 'Check cannot use len_diff.'
for BPF_MTU_CHK_SEGS flag. So right now for non-zero len_diff the user
will still get BPF_MTU_CHK_RET_SUCCESS if the current length check via
skb_gso_validate_network_len(skb, mtu) passes. If it cannot be checked,
maybe enforce len_diff == 0 for gso skbs on BPF_MTU_CHK_SEGS?

Ok. Do you want/think this can be enforced by the verifier or are you
simply requesting that the helper will return -EINVAL (or another errno)?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer