netdev - Re: [PATCH bpf] bpf: properly check TCP

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Date:   Mon, 25 Feb 2019 07:43:03 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        bpf <bpf@...r.kernel.org>, Lawrence Brakmo <brakmo@...com>
Subject: Re: [PATCH bpf] bpf: properly check TCP_CONGESTION optlen

On Mon, Feb 25, 2019 at 3:07 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 02/24/2019 12:11 AM, Alexei Starovoitov wrote:
> > On Sat, Feb 23, 2019 at 12:48:53PM -0800, Eric Dumazet wrote:
> >> On 02/23/2019 12:38 PM, Alexei Starovoitov wrote:
> >>> On Sat, Feb 23, 2019 at 11:07:09AM -0800, Eric Dumazet wrote:
> >>>> If caller of bpf_setsockopt() is silly passing a negative optlen
> >>>> bad things happen.
> >>>>
> >>>> Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
> >>>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >>>> Cc: Lawrence Brakmo <brakmo@...com>
> >>>> ---
> >>>>  net/core/filter.c | 5 +++--
> >>>>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/net/core/filter.c b/net/core/filter.c
> >>>> index f7d0004fc16096eb42ece3a6acf645540ee2326b..6a5d89464168f2f35f43986c1dbc0446c9390a3c 100644
> >>>> --- a/net/core/filter.c
> >>>> +++ b/net/core/filter.c
> >>>> @@ -4194,8 +4194,9 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
> >>>>                    char name[TCP_CA_NAME_MAX];
> >>>>                    bool reinit = bpf_sock->op > BPF_SOCK_OPS_NEEDS_ECN;
> >>>>
> >>>> -                  strncpy(name, optval, min_t(long, optlen,
> >>>> -                                              TCP_CA_NAME_MAX-1));
> >>>> +                  if (optlen < 0)
> >>>> +                          return -EINVAL;
> >>>> +                  strncpy(name, optval, min(optlen, TCP_CA_NAME_MAX - 1));
> >>>
> >>> Unnecessary.
> >>> The verifier guarantees that optlen > 0 because
> >>> static const struct bpf_func_proto bpf_setsockopt_proto = {
> >>>         .func           = bpf_setsockopt,
> >>> ...
> >>>         .arg5_type      = ARG_CONST_SIZE,
> >>> };
> >>
> >> Even on 32bit kernel ?
> >>
> >> The suspect thing to me is the min_t(long, ....)
> >>
> >> optlen is an integer, why do we need to promote to a long ?
> >
> > I think the code is actually fine as-is.
> > I bet it was copy pasted from do_tcp_setsockopt
> > where similar min_t(long) is used to match strncpy_from_user() declaration.
> > Here min_t(long) or min_t(int) or min() doesn't matter.
> > I would keep it as-is to avoid noisy patches.
>
> Max allowed input from verifier should be BPF_MAX_VAR_SIZ which is 1 << 29,
> but I totally agree that the bpf_setsockopt() and bpf_getsockopt() signature
> should change into u32 optlen as it's just confusing otherwise, same with the
> long in strncpy(). Agree with Alexei that this might have been a copy-paste
> kind of thing. Lawrence can probably clarify best?
>
> The same wrong assumption is in commit 1e215300f138 ("bpf: add TCP_SAVE_SYN/
> TCP_SAVED_SYN options for bpf_(set|get)sockopt") which tests for optlen <= 0.
> Neither can it be negative nor zero here due to ARG_CONST_SIZE. Given it's
> also confusing others, cleanup might still be worth considering. Maybe Lawrence
> can spin this into one of his next patches, if noone else gets to it first.

Yes, and considering the err_clear: error path does

memset(optval, 0, optlen);

This is even more confusing to think that optlen could be negative or
out of bound ;)