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)) { + return 0; + or if (write_len > skb_headlen(skb)) + write_len = skb_headlen(skb); + } 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) + if (!list_skb) + goto err_linearize; + else + err_len = true; +} ... +if (err_len) { + goto err_linearize; +} skb_get(skb); ... > > > > For a GSO_BY_FRAGS skb, there is no single gso_size, and this > pull > > > > may be entirely undetectable as long as frag_list != NULL? > > > > > > > > In function skb_segment_list(), it just handle udp fraglist gro packet. nr_frags will be 0 here. It records a SKB_GSO_DODGY in gso_type when doing partially eaten for fraglist in __pskb_pull_tail and in skb_segment() it will check and disable NETIF_F_SG. skb_segment could segment data as gso_size even if it is pulled into hearder skb. I am not sure if it can decode when frag_list is NULL or partially eaten as no BPF pulls illegal length for tcp packet. Our platfrom doesn't meet issues in skb_segment for tcp packet till now. > > > > > > > > skb_shinfo(skb)->frag_list = NULL; > > > > > > > > > > > > > > In absense of plugging the issue in BPF, dropping here is > the best > > > > > > > we can do indeed, I think. > > > > -- > > Maciej Żenczykowski, Kernel Networking Developer @ Google > >