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

[Willem de Bruijn]

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?

> 
> 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.