On 2022-01-31 23:19, John Fastabend wrote:
John Fastabend wrote:
Maxim Mikityanskiy wrote:
On 2022-01-25 09:54, John Fastabend wrote:
Maxim Mikityanskiy wrote:
The new helpers bpf_tcp_raw_{gen,check}_syncookie allow an XDP program
to generate SYN cookies in response to TCP SYN packets and to check
those cookies upon receiving the first ACK packet (the final packet of
the TCP handshake).
Unlike bpf_tcp_{gen,check}_syncookie these new helpers don't need a
listening socket on the local machine, which allows to use them together
with synproxy to accelerate SYN cookie generation.
Signed-off-by: Maxim Mikityanskiy <maximmi@xxxxxxxxxx>
Reviewed-by: Tariq Toukan <tariqt@xxxxxxxxxx>
---
[...]
+
+BPF_CALL_4(bpf_tcp_raw_check_syncookie, void *, iph, u32, iph_len,
+ struct tcphdr *, th, u32, th_len)
+{
+#ifdef CONFIG_SYN_COOKIES
+ u32 cookie;
+ int ret;
+
+ if (unlikely(th_len < sizeof(*th)))
+ return -EINVAL;
+
+ if (!th->ack || th->rst || th->syn)
+ return -EINVAL;
+
+ if (unlikely(iph_len < sizeof(struct iphdr)))
+ return -EINVAL;
+
+ cookie = ntohl(th->ack_seq) - 1;
+
+ /* Both struct iphdr and struct ipv6hdr have the version field at the
+ * same offset so we can cast to the shorter header (struct iphdr).
+ */
+ switch (((struct iphdr *)iph)->version) {
+ case 4:
Did you consider just exposing __cookie_v4_check() and __cookie_v6_check()?
No, I didn't, I just implemented it consistently with
bpf_tcp_check_syncookie, but let's consider it.
I can't just pass a pointer from BPF without passing the size, so I
would need some wrappers around __cookie_v{4,6}_check anyway. The checks
for th_len and iph_len would have to stay in the helpers. The check for
TCP flags (ACK, !RST, !SYN) could be either in the helper or in BPF. The
switch would obviously be gone.
I'm not sure you would need the len checks in helper, they provide
some guarantees I guess, but the void * is just memory I don't see
any checks on its size. It could be the last byte of a value for
example?
The verifier makes sure that the packet pointer and the size come
together in function parameters (see check_arg_pair_ok). It also makes
sure that the memory region defined by these two parameters is valid,
i.e. in our case it belongs to packet data.
Now that the helper got a valid memory region, its length is still
arbitrary. The helper has to check it's big enough to contain a TCP
header, before trying to access its fields. Hence the checks in the helper.
I suspect we need to add verifier checks here anyways to ensure we don't
walk off the end of a value unless something else is ensuring the iph
is inside a valid memory block.
The verifier ensures that the [iph; iph+iph_len) is valid memory, but
the helper still has to check that struct iphdr fits into this region.
Otherwise iph_len could be too small, and the helper would access memory
outside of the valid region.
The bottom line is that it would be the same code, but without the
switch, and repeated twice. What benefit do you see in this approach?
The only benefit would be to shave some instructions off the program.
XDP is about performance so I figure we shouldn't be adding arbitrary
stuff here. OTOH you're already jumping into a helper so it might
not matter at all.
From my side, I only see the ability to drop one branch at the expense
of duplicating the code above the switch (th_len and iph_len checks).
Just not sure you need the checks either, can you just assume the user
gives good data?
No, since the BPF program would be able to trick the kernel into reading
from an invalid location (see the explanation above).
My code at least has already run the code above before it would ever call
this helper so all the other bits are duplicate.
Sorry, I didn't quite understand this part. What "your code" are you
referring to?
Just that the XDP code I maintain has a if ipv4 {...} else ipv6{...}
structure
Same for my code (see the last patch in the series).
Splitting into two helpers would allow to drop the extra switch in the
helper, however:
1. The code will be duplicated for the checks.
2. It won't be consistent with bpf_tcp_check_syncookie (and all other
existing helpers - as far as I see, there is no split for IPv4/IPv6).
3. It's easier to misuse, e.g., pass an IPv6 header to the IPv4 helper.
This point is controversial, since it shouldn't pose any additional
security threat, but in my opinion, it's better to be foolproof. That
means, I'd add the IP version check even to the separate helpers, which
defeats the purpose of separating them.
Given these points, I'd prefer to keep it a single helper. However, if
you have strong objections, I can split it.
in it so could use a v4_check... and v6_check... then call
the correct version directly, removing the switch from the helper.
Do you think there could be a performance reason to drop out those
instructions or is it just hid by the hash itself. Also it seems
a bit annoying if user is calling multiple helpers and they keep
doing the same checks over and over.
