Re: [PATCH net] sctp: check stream reset info len before making reconf chunk

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Tue, Nov 14, 2017 at 04:27:41PM +0800, Xin Long wrote:
> On Tue, Nov 14, 2017 at 3:54 AM, Marcelo Ricardo Leitner
> <marcelo.leitner@gmail.com> wrote:
> > On Mon, Nov 13, 2017 at 11:15:40PM +0800, Xin Long wrote:
> >> On Mon, Nov 13, 2017 at 11:09 PM, Neil Horman <nhorman@tuxdriver.com> wrote:
> >> > On Mon, Nov 13, 2017 at 01:39:27PM +0800, Xin Long wrote:
> >> >> Now when resetting stream, if both in and out flags are set, the info
> >> >> len can reach:
> >> >>   sizeof(struct sctp_strreset_outreq) + SCTP_MAX_STREAM(65535) +
> >> >>   sizeof(struct sctp_strreset_inreq)  + SCTP_MAX_STREAM(65535)
> >> >> even without duplicated stream no, this value is far greater than the
> >> >> chunk's max size.
> >> >>
> >> >> _sctp_make_chunk doesn't do any check for this, which would cause the
> >> >> skb it allocs is huge, syzbot even reported a crash due to this.
> >> >>
> >> >> This patch is to check stream reset info len before making reconf
> >> >> chunk and return NULL if the len exceeds chunk's capacity.
> >> >>
> >> >> Fixes: cc16f00f6529 ("sctp: add support for generating stream reconf ssn reset request chunk")
> >> >> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> >> >> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> >> >> ---
> >> >>  net/sctp/sm_make_chunk.c | 7 +++++--
> >> >>  net/sctp/stream.c        | 8 +++++---
> >> >>  2 files changed, 10 insertions(+), 5 deletions(-)
> >> >>
> >> >> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
> >> >> index 514465b..a21328a 100644
> >> >> --- a/net/sctp/sm_make_chunk.c
> >> >> +++ b/net/sctp/sm_make_chunk.c
> >> >> @@ -3598,14 +3598,17 @@ struct sctp_chunk *sctp_make_strreset_req(
> >> >>       __u16 stream_len = stream_num * 2;
> >
> > Unrelated, but.. won't stream_len overflow if stream_num >= 32768?
> > When called form sctp_send_reset_streams() I don't see anything
> > restricting it to such range.
> right.
> 
> >
> >> >>       struct sctp_strreset_inreq inreq;
> >> >>       struct sctp_chunk *retval;
> >> >> -     __u16 outlen, inlen;
> >> >> +     int outlen, inlen;
> >> >>
> >> >>       outlen = (sizeof(outreq) + stream_len) * out;
> >> >>       inlen = (sizeof(inreq) + stream_len) * in;
> >> >>
> >> >> +     if (outlen + inlen > SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_chunkhdr))
> >> >> +             return ERR_PTR(-EINVAL);
> >> >> +
> >> > Why all the ERR_PTR manipulations here?  Just returning NULL, like the fuction
> >> > has been doing is sufficient to set ENOMEM at both call sites
> >> I don't like ERR_PTR handling here either,
> >> But it shouldn't be ENOMEM, should it ?
> >>
> >> It may confuse users, but I'm also ok to let it just return
> >> ENOMEM as you wish. wdyt ?
> >
> > Returning ENOMEM in the above error can be misleading. It's not that
> > we cannot allocate it, it's that it won't fit the packet no matter how
> > much memory we add to the system.
> right.
> 
> let's move the check into sctp_send_reset_streams()
> 
> I believe this one fixes them both:
> @@ -139,15 +139,31 @@ int sctp_send_reset_streams(struct sctp_association *asoc,
> 
>         str_nums = params->srs_number_streams;
>         str_list = params->srs_stream_list;
> -       if (out && str_nums)
> -               for (i = 0; i < str_nums; i++)
> -                       if (str_list[i] >= stream->outcnt)
> -                               goto out;
> +       if (str_nums) {
> +               int param_len = 0;
> 
> -       if (in && str_nums)
> -               for (i = 0; i < str_nums; i++)
> -                       if (str_list[i] >= stream->incnt)
> -                               goto out;
> +               if (out) {
> +                       for (i = 0; i < str_nums; i++)
> +                               if (str_list[i] >= stream->outcnt)
> +                                       goto out;
> +
> +                       param_len = str_nums * 2 +
                                 sizeof(__u16) --^

> +                                   sizeof(struct sctp_strreset_outreq);
> +               }
> +
> +               if (in) {
> +                       for (i = 0; i < str_nums; i++)
> +                               if (str_list[i] >= stream->incnt)
> +                                       goto out;
> +
> +                       param_len += str_nums * 2 +
                                  sizeof(__u16) --^

> +                                    sizeof(struct sctp_strreset_inreq);
> +               }
> +
> +               if (param_len > SCTP_MAX_CHUNK_LEN -
> +                               sizeof(struct sctp_reconf_chunk))
> +                       goto out;
> +       }
> 
> 
> and int this fix,  it's good to do all checks only when str_nums !=0.
> --
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