Message-ID: <CAKH8qBvUv_OwjFA70JQfL-rET662okH87QYyeivbybCPwCEJEQ@mail.gmail.com>
Date:   Mon, 15 Jun 2020 09:41:38 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH bpf v3 1/2] bpf: don't return EINVAL from {get,set}sockopt
 when optlen > PAGE_SIZE

On Fri, Jun 12, 2020 at 8:50 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
[ .. ]
> > > It's probably ok, but makes me uneasy about verifier consequences.
> > > ctx->optval is PTR_TO_PACKET and it's a valid pointer from verifier pov.
> > > Do we have cases already where PTR_TO_PACKET == PTR_TO_PACKET_END ?
> > > I don't think we have such tests. I guess bpf prog won't be able to read
> > > anything and nothing will crash, but having PTR_TO_PACKET that is
> > > actually NULL would be an odd special case to keep in mind for everyone
> > > who will work on the verifier from now on.
> > >
> > > Also consider bpf prog that simply reads something small like 4 bytes.
> > > IP_FREEBIND sockopt (like your selftest in the patch 2) will have
> > > those 4 bytes, so it's natural for the prog to assume that it can read it.
> > > It will have
> > > p = ctx->optval;
> > > if (p + 4 > ctx->optval_end)
> > >  /* goto out and don't bother logging, since that never happens */
> > > *(u32*)p;
> > >
> > > but 'clever' user space would pass long optlen and prog suddenly
> > > 'not seeing' the sockopt. It didn't crash, but debugging would be
> > > surprising.
> > >
> > > I feel it's better to copy the first 4k and let the program see it.
> > Agreed with the IP_FREEBIND example wrt observability, however it's
> > not clear what to do with the cropped buffer if the bpf program
> > modifies it.
> >
> > Consider that huge iptables setsockopts where the usespace passes
> > PAGE_SIZE*10 optlen with real data and bpf prog sees only part of it.
> > Now, if the bpf program modifies the buffer (say, flips some byte), we
> > are back to square one. We either have to silently discard that buffer
> > or reallocate/merge. My reasoning with data == NULL, is that at least
> > there is a clear signal that the program can't access the data (and
> > can look at optlen to see if the original buffer is indeed non-zero
> > and maybe deny such requests?).
> > At this point I'm really starting to think that maybe we should just
> > vmalloc everything that is >PAGE_SIZE and add a sysclt to limit an
> > upper bound :-/
> > I'll try to think about this a bit more over the weekend.
>
> Yeah. Tough choices.
> We can also detect in the verifier whether program accessed ctx->optval
> and skip alloc/copy if program didn't touch it, but I suspect in most
> case the program would want to read it.
> I think vmallocing what optlen said is DoS-able. It's better to
> stick with single page.
> Let's keep brainstorming.
Btw, can we use sleepable bpf for that? As in, do whatever I suggested
in these patches (don't expose optval>PAGE_SIZE via context), but add
a new helper where you can say 'copy x bytes from y offset of the
original optval' (the helper will do sleepable copy_form_user).
That way we have a clean signal to the BPF that the value is too big
(optval==optval_end==NULL) and the user can fallback to the helper to
inspect the value. We can also provide another helper to export new
value for this case.
WDYT?

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