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

[Lena Wang (王娜)]

On Mon, 2024-04-15 at 16:53 -0400, Willem de Bruijn wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  shiming.cheng@ wrote:
> > From: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> > 
> > A GRO packet without fraglist is crashed and backtrace is as below:
> >  [ 1100.812205][    C3] CPU: 3 PID: 0 Comm: swapper/3 Tainted:
> > G        W  OE      6.6.17-android15-0-g380371ea9bf1 #1
> >  [ 1100.812317][    C3]  __udp_gso_segment+0x298/0x4d4
> >  [ 1100.812335][    C3]  __skb_gso_segment+0xc4/0x120
> >  [ 1100.812339][    C3]  udp_rcv_segment+0x50/0x134
> >  [ 1100.812344][    C3]  udp_queue_rcv_skb+0x74/0x114
> >  [ 1100.812348][    C3]  udp_unicast_rcv_skb+0x94/0xac
> >  [ 1100.812358][    C3]  udp_rcv+0x20/0x30
> > 
> > The reason that the packet loses its fraglist is that in ingress
> bpf
> > it makes a test pull with to make sure it can read packet headers
> > via direct packet access: In bpf_progs/offload.c
> > try_make_writable -> bpf_skb_pull_data -> pskb_may_pull ->
> > __pskb_pull_tail  This operation pull the data in fraglist into
> linear
> > and set the fraglist to null.
> 
> What is the right behavior from BPF with regard to SKB_GSO_FRAGLIST
> skbs?
> 
> Some, like SCTP, cannot be linearized ever, as the do not have a
> single gso_size.
> 
> Should this BPF operation just fail?
> 
In most situation for big gso size packet, it indeed fails but BPF
doesn't check the result. It seems the udp GRO packet can't be pulled/
trimed/condensed or else it can't be segmented correctly.

As the BPF function comments it doesn't matter if the data pull failed 
or pull less. It just does a blind best effort pull.

A patch to modify bpf pull length is upstreamed to Google before and
below are part of Google BPF expert maze's reply:
maze@xxxxxxxxxx<maze@xxxxxxxxxx> #5Apr 13, 2024 02:30AM
I *think* if that patch fixes anything, then it's really proving that
there's a bug in the kernel that needs to be fixed instead.
It should be legal to call try_make_writable(skb, X) with *any* value
of X.

I add maze in loop and we could start more discussion here.

> > 
> > 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;
> > +
> >  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.
>