Re: [PATCH net] udp: fix segmentation crash for GRO packet without fraglist

[Maciej Żenczykowski]

On Wed, Apr 17, 2024 at 7:53 PM Lena Wang (王娜) <Lena.Wang@xxxxxxxxxxxx> wrote:
>
> On Wed, 2024-04-17 at 15:48 -0400, Willem de Bruijn wrote:
> >
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >  Lena Wang (王娜) wrote:
> > > On Tue, 2024-04-16 at 19:14 -0400, Willem de Bruijn wrote:
> > > >
> > > > External email : Please do not click links or open attachments
> > until
> > > > you have verified the sender or the content.
> > > >  > > > > Personally, I think bpf_skb_pull_data() should have
> > > > automatically
> > > > > > > > (ie. in kernel code) reduced how much it pulls so that it
> > > > would pull
> > > > > > > > headers only,
> > > > > > >
> > > > > > > That would be a helper that parses headers to discover
> > header
> > > > length.
> > > > > >
> > > > > > Does it actually need to?  Presumably the bpf pull function
> > could
> > > > > > notice that it is
> > > > > > a packet flagged as being of type X (UDP GSO FRAGLIST) and
> > reduce
> > > > the pull
> > > > > > accordingly so that it doesn't pull anything from the non-
> > linear
> > > > > > fraglist portion???
> > > > > >
> > > > > > I know only the generic overview of what udp gso is, not any
> > > > details, so I am
> > > > > > assuming here that there's some sort of guarantee to how
> > these
> > > > packets
> > > > > > are structured...  But I imagine there must be or we wouldn't
> > be
> > > > hitting these
> > > > > > issues deeper in the stack?
> > > > >
> > > > > Perhaps for a packet of this type we're already guaranteed the
> > > > headers
> > > > > are in the linear portion,
> > > > > and the pull should simply be ignored?
> > > > >
> > > > > >
> > > > > > > Parsing is better left to the BPF program.
> > > >
> > > > I do prefer adding sanity checks to the BPF helpers, over having
> > to
> > > > add then in the net hot path only to protect against dangerous
> > BPF
> > > > programs.
> > > >
> > > Is it OK to ignore or decrease pull length for udp gro fraglist
> > packet?
> > > It could save the normal packet and sent to user correctly.
> > >
> > > In common/net/core/filter.c
> > > static inline int __bpf_try_make_writable(struct sk_buff *skb,
> > >               unsigned int write_len)
> > > {
> > > +if (skb_is_gso(skb) && (skb_shinfo(skb)->gso_type &
> > > +(SKB_GSO_UDP  |SKB_GSO_UDP_L4)) {
> >
> > The issue is not with SKB_GSO_UDP_L4, but with SKB_GSO_FRAGLIST.
> >
> Current in kernel just UDP uses SKB_GSO_FRAGLIST to do GRO. In
> udp_offload.c udp4_gro_complete gso_type adds "SKB_GSO_FRAGLIST|
> SKB_GSO_UDP_L4". Here checking these two flags is to limit the packet
> as "UDP + need GSO + fraglist".
>
> We could remove SKB_GSO_UDP_L4 check for more packet that may addrive
> skb_segment_list.
>
> > > +return 0;
> >
> > Failing for any pull is a bit excessive. And would kill a sane
> > workaround of pulling only as many bytes as needed.
> >
> > > +     or if (write_len > skb_headlen(skb))
> > > +write_len = skb_headlen(skb);
> >
> > Truncating requests would be a surprising change of behavior
> > for this function.
> >
> > Failing for a pull > skb_headlen is arguably reasonable, as
> > the alternative is that we let it go through but have to drop
> > the now malformed packets on segmentation.
> >
> >
> Is it OK as below?
>
> In common/net/core/filter.c
> static inline int __bpf_try_make_writable(struct sk_buff *skb,
>               unsigned int write_len)
> {
> +       if (skb_is_gso(skb) && (skb_shinfo(skb)->gso_type &
> +               SKB_GSO_FRAGLIST) && (write_len > skb_headlen(skb))) {
> +               return 0;

please limit write_len to skb_headlen() instead of just returning 0

> +       }
>         return skb_ensure_writable(skb, write_len);
> }
>
> > > +}
> > > return skb_ensure_writable(skb, write_len);
> > > }
> > >
> > >
> > > > In this case, it would be detecting this GSO type and failing the
> > > > operation if exceeding skb_headlen().
> > > > > > >
> > > > > > > > and not packet content.
> > > > > > > > (This is assuming the rest of the code isn't ready to
> > deal
> > > > with a longer pull,
> > > > > > > > which I think is the case atm.  Pulling too much, and
> > then
> > > > crashing or forcing
> > > > > > > > the stack to drop packets because of them being malformed
> > > > seems wrong...)
> > > > > > > >
> > > > > > > > In general it would be nice if there was a way to just
> > say
> > > > pull all headers...
> > > > > > > > (or possibly all L2/L3/L4 headers)
> > > > > > > > You in general need to pull stuff *before* you've even
> > looked
> > > > at the packet,
> > > > > > > > so that you can look at the packet,
> > > > > > > > so it's relatively hard/annoying to pull the correct
> > length
> > > > from bpf
> > > > > > > > code itself.
> > > > > > > >
> > > > > > > > > > > BPF needs to modify a proper length to do pull
> > data.
> > > > However kernel
> > > > > > > > > > > should also improve the flow to avoid crash from a
> > bpf
> > > > function
> > > > > > > > > > call.
> > > > > > > > > > > As there is no split flow and app may not decode
> > the
> > > > merged UDP
> > > > > > > > > > packet,
> > > > > > > > > > > we should drop the packet without fraglist in
> > > > skb_segment_list
> > > > > > > > > > here.
> > > > > > > > > > >
> > > > > > > > > > > Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist
> > > > chaining.")
> > > > > > > > > > > Signed-off-by: Shiming Cheng <
> > > > shiming.cheng@xxxxxxxxxxxx>
> > > > > > > > > > > Signed-off-by: Lena Wang <lena.wang@xxxxxxxxxxxx>
> > > > > > > > > > > ---
> > > > > > > > > > >  net/core/skbuff.c | 3 +++
> > > > > > > > > > >  1 file changed, 3 insertions(+)
> > > > > > > > > > >
> > > > > > > > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > > > > > > > > > index b99127712e67..f68f2679b086 100644
> > > > > > > > > > > --- a/net/core/skbuff.c
> > > > > > > > > > > +++ b/net/core/skbuff.c
> > > > > > > > > > > @@ -4504,6 +4504,9 @@ struct sk_buff
> > > > *skb_segment_list(struct
> > > > > > > > > > sk_buff *skb,
> > > > > > > > > > >  if (err)
> > > > > > > > > > >  goto err_linearize;
> > > > > > > > > > >
> > > > > > > > > > > +if (!list_skb)
> > > > > > > > > > > +goto err_linearize;
> > > > > > > > > > > +
> > > > > > >
> > > > > > > This would catch the case where the entire data frag_list
> > is
> > > > > > > linearized, but not a pskb_may_pull that only pulls in part
> > of
> > > > the
> > > > > > > list.
> > > > > > >
> > > > > > > Even with BPF being privileged, the kernel should not crash
> > if
> > > > BPF
> > > > > > > pulls a FRAGLIST GSO skb.
> > > > > > >
> > > > > > > But the check needs to be refined a bit. For a UDP GSO
> > packet,
> > > > I
> > > > > > > think gso_size is still valid, so if the head_skb length
> > does
> > > > not
> > > > > > > match gso_size, it has been messed with and should be
> > dropped.
> > > > > > >
> > > Is it OK as below? Is it OK to add log to record the error for easy
> > > checking issue.
> > >
> > > In net/core/skbuff.c skb_segment_list
> > > +unsigned int mss = skb_shinfo(head_skb)->gso_size;
> > > +bool err_len = false;
> > >
> > > +if ( mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
> > > +pr_err("skb is dropped due to messed data. gso size:%d,
> > > +hdrlen:%d", mss, skb_headlen(head_skb)
> >
> > Such logs should always be rate limited. But no need to log cases
> > where we well understood how we get there.
> >
> > I would stick with one approach: either in the BPF func or in
> > segmentation, not both. And then I find BPF preferable, as explained
> > before.
> >
> OK, we try make a patch in BPF func.
>
> > > +if (!list_skb)
> > > +goto err_linearize;
> > > +else
> > > +err_len = true;
> > > +}
> > >
> > > ...
> > > +if (err_len) {
> > > +goto err_linearize;
> > > +}
> > >
> > > skb_get(skb);
> > > ...

--
Maciej Żenczykowski, Kernel Networking Developer @ Google