Date:   Mon, 4 Mar 2019 17:42:30 +0000
From:   Martin Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <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 Mon, Mar 04, 2019 at 10:33:46AM +0100, Daniel Borkmann wrote:
> On 03/02/2019 09:21 PM, Martin Lau wrote:
> > On Sat, Mar 02, 2019 at 10:03:03AM -0800, Alexei Starovoitov wrote:
> >> 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 ?
> > I like this idea.  Many thanks for thinking it through!
> > 
> > Allowing bpf_sk_release(tp), no need to call bpf_sk_release() on ptr
> > returned from bpf_get_listener_sock(sk) and keep one reg->id.
> > 
> > I think it should work.  I will rework the patches.
> 
> Agree, makes sense, that seems much better fix.
While I was working on this change, based on the code, one issue I saw is:

if the bpf prog does this:

sk = bpf_sk_lookup_tcp();
/* if (!sk) ... */
fullsock = bpf_sk_fullsock(sk);
if (!fullsock) {
	bpf_sk_release(sk); /* Fail. sk_reg->id not found in ref state */
	return 0;
}

The bpf_sk_release(sk) failed because the reference state has already
been released by "release_reference_state(state, fullsock_reg->id)" during
"if (!fullsock) /* handled by mark_ptr_or_null_regs(is_null == true) */"
Logically, I think bpf_sk_release(sk) should not fail regardless of
bpf_sk_fullsock() doing sk_to_full_sk() or not.

bpf_sk_fullsock() could disallow PTR_TO_SOCKET or PTR_TO_TCP_SOCK but that
would be weird.

I think we still need two id.  May be rename the refcount_id proposed in
this patch to ref_obj_id which is the original refcounted object id.

If the sk_to_full_sk() is removed from bpf_sk_fullsock() and bpf_tcp_sock(),
these two helpers become a simple cast (i.e. either return the same pointer
or NULL).  Then bpf_sk_release(fullsock) and bpf_sk_release(tp) could work:

- When is_null == true, release_reference_state(state, reg->id) is called.
- During bpf_sk_release(fullsock), release_reference(env, reg->ref_obj_id)
  is called so that sk, fullsock and tp with the same ref_obj_id will
  be mark_reg_unknown().

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