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

[Willem de Bruijn]

Daniel Borkmann wrote:
> On 4/26/24 11:52 AM, Lena Wang (王娜) wrote:
> [...]
> >>>  From 301da5c9d65652bac6091d4cd64b751b3338f8bb Mon Sep 17 00:00:00
> >> 2001
> >>> From: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> >>> Date: Wed, 24 Apr 2024 13:42:35 +0800
> >>> Subject: [PATCH net] net: prevent BPF pulling SKB_GSO_FRAGLIST skb
> >>>
> >>> A SKB_GSO_FRAGLIST skb can't be pulled data
> >>> from its fraglist as it may result an invalid
> >>> segmentation or kernel exception.
> >>>
> >>> For such structured skb we limit the BPF pulling
> >>> data length smaller than skb_headlen() and return
> >>> error if exceeding.
> >>>
> >>> 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/filter.c | 5 +++++
> >>>   1 file changed, 5 insertions(+)
> >>>
> >>> diff --git a/net/core/filter.c b/net/core/filter.c
> >>> index 8adf95765cdd..8ed4d5d87167 100644
> >>> --- a/net/core/filter.c
> >>> +++ b/net/core/filter.c
> >>> @@ -1662,6 +1662,11 @@ static DEFINE_PER_CPU(struct bpf_scratchpad,
> >>> bpf_sp);
> >>>   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 -ENOMEM;
> >>> +}
> >>>   return skb_ensure_writable(skb, write_len);
> 
> Dumb question, but should this guard be more generically part of skb_ensure_writable()
> internals, presumably that would be inside pskb_may_pull_reason(), or only if we ever
> see more code instances similar to this?

Good point. Most callers of skb_ensure_writable correctly pull only
headers, so wouldn't cause this problem. But it also adds coverage to
things like tc pedit.