On 1/25/21 9:41 AM, Jesper Dangaard Brouer wrote:
On Sat, 23 Jan 2021 02:35:41 +0100
Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
+ * 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**
+ *
*/
[...]
+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)?
Simple -EINVAL should be fine in this case. Generally, we can detect this from
verifier side but I don't think the extra complexity is worth it especially given
this is dependent on BPF_MTU_CHK_SEGS and otherwise can be non-zero.
Thanks,
Daniel
