netdev - Re: [PATCH v3 bpf-next 1/2] bpf: Fix bpf_tcp_sock and bpf_sk_fullsock issue related to bpf_sk

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Message-ID: <20190302180301.liwjbsz7qlsg33ov@ast-mbp.dhcp.thefacebook.com>
Date:   Sat, 2 Mar 2019 10:03:03 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [PATCH v3 bpf-next 1/2] bpf: Fix bpf_tcp_sock and
 bpf_sk_fullsock issue related to bpf_sk_release

On Sat, Mar 02, 2019 at 08:10:10AM -0800, Martin KaFai Lau wrote:
> Lorenz Bauer [thanks!] reported that a ptr returned by bpf_tcp_sock(sk)
> can still be accessed after bpf_sk_release(sk).
> Both bpf_tcp_sock() and bpf_sk_fullsock() have the same issue.
> This patch addresses them together.
> 
> A simple reproducer looks like this:
> 
> sk = bpf_sk_lookup_tcp();
> /* if (!sk) ... */
> tp = bpf_tcp_sock(sk);
> /* if (!tp) ... */
> bpf_sk_release(sk);
> snd_cwnd = tp->snd_cwnd; /* oops! The verifier does not complain. */
> 
> The problem is the verifier did not scrub the register's states of
> the tcp_sock ptr (tp) after bpf_sk_release(sk).
> 
> [ Note that when calling bpf_tcp_sock(sk), the sk is not always
>   refcount-acquired. e.g. bpf_tcp_sock(skb->sk). The verifier works
>   fine for this case. ]
> 
> Currently, the verifier does not track if a helper's return ptr (in REG_0)
> is "carry"-ing one of its argument's refcount status. To carry this info,
> the reg1->id needs to be stored in reg0.  The reg0->id has already
> been used for NULL checking purpose.  Hence, a new "refcount_id"
> is needed in "struct bpf_reg_state".
> 
> With refcount_id, when bpf_sk_release(sk) is called, the verifier can scrub
> all reg states which has a refcount_id match.  It is done with the changes
> in release_reg_references().
> 
> When acquiring and releasing a refcount, the reg->id is still used.
> Hence, we cannot do "bpf_sk_release(tp)" in the above reproducer
> example.

I think the choice of returning listener full sock from req sock
in sk_to_full_sk() was a wrong one.
It seems better to make semantics of bpf_tcp_sock() and bpf_sk_fullsock() as 
always type cast or null.
And have a separate helper for req socket that returns inet_reqsk(sk)->rsk_listener.

Then it will be ok to call bpf_sk_release(tp) when tp came from bpf_sk_lookup_tcp.
The verifier will know that it's the case because its ID will be in acquired_refs.

The additional refcount_id won't be necessary.
bpf_sk_fullsock() and bpf_tcp_sock() will not call sk_to_full_sk
and the verifier will be copying reg1->id into reg0->id.

In release_reference() the verifier will do
  if (regs[i].id == id)
    mark_reg_unknown(env, regs, i);
for all socket types.

release_reference_state() will stay as-is.

imo such logic will be easier to follow.

This implicit sk_to_full_sk() makes the whole thing much harder for the verifier
and for the bpf program writers.

The new bpf_get_listener_sock(sk) doesn't have to copy ID from reg1 to reg0
since req socket will not be returned from bpf_sk_lookup_tcp and its ID
will not be stored in acuired_refs.

Does it make sense ?