RE: [PATCH bpf-next v2 2/3] bpf: Add helpers to issue and check SYN cookies in XDP

24 Feb 2022

      ...
-----Original Message-----
From: Kumar Kartikeya Dwivedi memxor@gmail.com
Sent: 21 February, 2022 17:22
To: Maxim Mikityanskiy maximmi@nvidia.com
Cc: Toke Høiland-Jørgensen toke@toke.dk; John Fastabend
john.fastabend@gmail.com; bpf@vger.kernel.org; Alexei Starovoitov
ast@kernel.org; Daniel Borkmann daniel@iogearbox.net; Andrii Nakryiko
andrii@kernel.org; netdev@vger.kernel.org; Tariq Toukan
tariqt@nvidia.com; Martin KaFai Lau kafai@fb.com; Song Liu
songliubraving@fb.com; Yonghong Song yhs@fb.com; KP Singh
kpsingh@kernel.org; David S. Miller davem@davemloft.net; Jakub Kicinski
kuba@kernel.org; Petar Penkov ppenkov@google.com; Lorenz Bauer
lmb@cloudflare.com; Eric Dumazet edumazet@google.com; Hideaki YOSHIFUJI
yoshfuji@linux-ipv6.org; David Ahern dsahern@kernel.org; Shuah Khan
shuah@kernel.org; Jesper Dangaard Brouer hawk@kernel.org; Nathan
Chancellor nathan@kernel.org; Nick Desaulniers ndesaulniers@google.com;
Joe Stringer joe@cilium.io; Florent Revest revest@chromium.org; linux-
kselftest@vger.kernel.org; Florian Westphal fw@strlen.de
Subject: Re: [PATCH bpf-next v2 2/3] bpf: Add helpers to issue and check SYN
cookies in XDP
On Mon, Feb 21, 2022 at 07:56:28PM IST, Maxim Mikityanskiy wrote:
...
On 2022-02-04 16:08, Toke Høiland-Jørgensen wrote:
...
John Fastabend john.fastabend@gmail.com writes:
...
Maxim Mikityanskiy wrote:
...
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@nvidia.com
> > > > Reviewed-by: Tariq Toukan tariqt@nvidia.com
> > > > ---
> > >
> > > [...]
> > >
> > > > +
> > > > +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.
Thanks for the details this all makes sense. See response to
other mail about adding new types. Replied to the wrong email
but I think the context is not lost.
Keeping my reply here in an attempt to de-fork :)
...
...
...
> >
> > 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:

The code will be duplicated for the checks.

See response wrt PTR_TO_IP, PTR_TO_TCP types.
So about that (quoting some context from your other email):
...
We could have some new mem types, PTR_TO_IPV4, PTR_TO_IPv6, and
PTR_TO_TCP.
...
...
...
Then we simplify the helper signatures to just,
bpf_tcp_raw_check_syncookie_v4(iph, tcph);
   bpf_tcp_raw_check_syncookie_v6(iph, tcph);
And the verifier "knows" what a v4/v6 header is and does the mem
check at verification time instead of run time.
I think this could probably be achieved with PTR_TO_BTF arguments to the
helper (if we define appropriate struct types that the program can use
for each header type)?
I get the following error when I try to pass the headers from packet data
to
...
a helper that accepts ARG_PTR_TO_BTF_ID:
; value = bpf_tcp_raw_gen_syncookie_ipv4(hdr->ipv4, hdr->tcp,
297: (79) r1 = *(u64 *)(r10 -80)      ; R1_w=pkt(id=0,off=14,r=74,imm=0)
R10=fp0
298: (79) r2 = *(u64 *)(r10 -72)      ;
R2_w=pkt(id=5,off=14,r=74,umax_value=60,var_off=(0x0; 0x3c)) R10=fp0
299: (bc) w3 = w9                     ;
R3_w=invP(id=0,umin_value=20,umax_value=60,var_off=(0x0; 0x3c))
R9=invP(id=0,umin_value=20,umax_value=60,var_off=(0x0; 0x3c))
300: (85) call bpf_tcp_raw_gen_syncookie_ipv4#192
R1 type=pkt expected=ptr_
processed 317 insns (limit 1000000) max_states_per_insn 0 total_states 23
peak_states 23 mark_read 12
-- END PROG LOAD LOG --
It looks like the verifier doesn't currently support such type conversion.
Could you give any clue what is needed to add this support? Is it enough
to
...
extend compatible_reg_types, or should more checks be added anywhere?
I think what he meant was getting the size hint from the function prototype.
In
case of kfunc we do it by resolving type size from BTF, for the PTR_TO_MEM
case
when a size argument is missing. For helper, you can add a field to indicate
the
constant size hint in the bpf_func_proto, and then in check_func_arg
directly do
the equivalent check_helper_mem_access for arg_type_is_mem_ptr block if such
a
hint is set, instead of delaying it till check_mem_size_reg call when the
next
arg_type_is_mem_size block is executed.
Then you can have two helpers with same argument types but different size
hint
values for the header argument, so you wouldn't need an extra mem size
parameter.
You may also want to disallow setting both the size hint and next argument
as
ARG_CONST_SIZE.
Thanks, I implemented your suggestion as a new feature of the verifier,
and it works.
I'm ready to respin the series, I've split my new helpers, but splitting
the existing helpers won't be part of resubmission, because it is out of
scope of changes I intended to push. It can be added later as an
improvement, though.
...
...
Alternatively, I can revert to ARG_PTR_TO_MEM and do size checks in
runtime
...
in the helper.
[...]
--
Kartikeya

2024

2023

2022

2021

2020

2019

2018

2017

RE: [PATCH bpf-next v2 2/3] bpf: Add helpers to issue and check SYN cookies in XDP