Re: [RFC PATCH v3 02/20] tcp: don't allow non-devmem originated ppiov

Mina Almasry
2024-01-02T16:12:05Z

On Tue, Dec 19, 2023 at 5:34 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 12/19/23 23:24, Mina Almasry wrote:
> > On Tue, Dec 19, 2023 at 1:04 PM David Wei <dw@davidwei.uk> wrote:
> >>
> >> From: Pavel Begunkov <asml.silence@gmail.com>
> >>
> >> NOT FOR UPSTREAM
> >>
> >> There will be more users of struct page_pool_iov, and ppiovs from one
> >> subsystem must not be used by another. That should never happen for any
> >> sane application, but we need to enforce it in case of bufs and/or
> >> malicious users.
> >>
> >> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >> Signed-off-by: David Wei <dw@davidwei.uk>
> >> ---
> >> net/ipv4/tcp.c | 7 +++++++
> >> 1 file changed, 7 insertions(+)
> >>
> >> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> >> index 33a8bb63fbf5..9c6b18eebb5b 100644
> >> --- a/net/ipv4/tcp.c
> >> +++ b/net/ipv4/tcp.c
> >> @@ -2384,6 +2384,13 @@ static int tcp_recvmsg_devmem(const struct sock *sk, const struct sk_buff *skb,
> >> }
> >>
> >> ppiov = skb_frag_page_pool_iov(frag);
> >> +
> >> + /* Disallow non devmem owned buffers */
> >> + if (ppiov->pp->p.memory_provider != PP_MP_DMABUF_DEVMEM) {
> >> + err = -ENODEV;
> >> + goto out;
> >> + }
> >> +
> >
> > Instead of this, I maybe recommend modifying the skb->dmabuf flag? My
> > mental model is that flag means all the frags in the skb are
>
> That's a good point, we need to separate them, and I have it in my
> todo list.
>
> > specifically dmabuf, not general ppiovs or net_iovs. Is it possible to
> > add skb->io_uring or something?
>
> ->io_uring flag is not feasible, converting ->devmem into a type
> {page,devmem,iouring} is better but not great either.
>
> > If that bloats the skb headers, then maybe we need another place to
> > put this flag. Maybe the [page_pool|net]_iov should declare whether
> > it's dmabuf or otherwise, and we can check frag[0] and assume all
>
> ppiov->pp should be enough, either not mixing buffers from different
> pools or comparing pp->ops or some pp->type.
>
> > frags are the same as frag0.
>
> I think I like this one the most. I think David Ahern mentioned
> before, but would be nice having it on per frag basis and kill
> ->devmem flag. That would still stop collapsing if frags are
> from different pools or so.
>

This sounds reasonable to me. I'll look into applying this change to
my next devmem TCP RFC, thanks.

> > But IMO the page pool internals should not leak into the
> > implementation of generic tcp stack functions.
>
> --
> Pavel Begunkov

--
Thanks,
Mina