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

[Willem de Bruijn]

Lena Wang (王娜) wrote:
> On Tue, 2024-04-23 at 14:35 -0400, Willem de Bruijn wrote:
> >  	 
> > External email : Please do not click links or open attachments until
> > you have verified the sender or the content.
> >  > Hi Willem,
> > > As the discussion, is it OK for the patch below?
> > 
> > Thanks for iterating on this.
> > 
> > I would like the opinion also of the fraglist and UDP GRO experts.
> >  
> > Yes, I think both
> > 
> > - protecting skb_segment_list against clearly illegal fraglist
> > packets, and
> > - blocking BPF from constructing such packets
> > 
> > are worthwhile stable fixes. I believe they should be two separate
> > patches. Both probably with the same Fixes tag: 3a1296a38d0c
> > ("net: Support GRO/GSO fraglist chaining").
> > 
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 3a6110ea4009..abc6029c8eef 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -1655,6 +1655,11 @@ 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;
> > > +       }
> > > +
> > 
> > Indentation looks off, but I agree with the logic.
> > 
> >     if (skb_is_gso(skb) &&
> >         (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) &&
> >          (write_len > skb_headlen(skb)))
> > 
> > >         return skb_ensure_writable(skb, write_len);
> > >  }
> > > 
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index 73b1e0e53534..2e90534c1a1e 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -4036,9 +4036,11 @@ struct sk_buff *skb_segment_list(struct
> > sk_buff
> > > *skb,
> > >         unsigned int tnl_hlen = skb_tnl_header_len(skb);
> > >         unsigned int delta_truesize = 0;
> > >         unsigned int delta_len = 0;
> > > +       unsigned int mss = skb_shinfo(skb)->gso_size;
> > >         struct sk_buff *tail = NULL;
> > >         struct sk_buff *nskb, *tmp;
> > >         int len_diff, err;
> > > +       bool err_len = false;
> > > 
> > >         skb_push(skb, -skb_network_offset(skb) + offset);
> > > 
> > > @@ -4047,6 +4049,14 @@ struct sk_buff *skb_segment_list(struct
> > sk_buff
> > > *skb,
> > >         if (err)
> > >                 goto err_linearize;
> > > 
> > > +       if (mss != GSO_BY_FRAGS && mss != skb_headlen(skb)) {
> > > +               if (!list_skb) {
> > > +                       goto err_linearize;
> > 
> > The label no longer truly covers the meaning.
> > 
> > But that is already true since the above (second) jump was added in
> > commit c329b261afe7 ("net: prevent skb corruption on frag list
> > segmentation").
> > 
> > Neither needs the kfree_skb_list, as skb->next is not assigned to
> > until the loop. Can just return ERR_PTR(-EFAULT)?
> > 
> > > +               } else {
> > > +                       err_len = true;
> > > +               }
> > > +       }
> > > +
> > 
> > Why the branch? Might as well always fail immediately?
> > 
> Hi Willem,
> Thanks for your guidance.
> You are right. There is no need for another branch as fraglist
> could be freeed in kfree_skb.
> Could I git send mail wo patches as below?
> 
> From 933237400c0e2fa997470b70ff0e407996fa239c 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 pull GROed skb's fraglist
> 
> A GROed skb with fraglist can't be pulled data

Please use the specific label: SKB_GSO_FRAGLIST skb

> from its fraglist as it may result a invalid
> segmentation or kernel exception.
> 
> For such structured skb we limit the BPF pull
> 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);
>  }
>  
> -- 
> 2.18.0
> 
> 
> From 2d0729b20cf810ba1b31e046952c1cd78f295ca3 Mon Sep 17 00:00:00 2001
> From: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> Date: Wed, 24 Apr 2024 14:43:45 +0800
> Subject: [PATCH net] net: drop GROed skb pulled from fraglist
> 
> A GROed skb with fraglist maybe pulled by BPF
> or other ways. It can't be trusted at all even
> if one byte is pulled and should be dropped
> on segmentation.

This paraphrases my comment. It is better to spell it out:

An SKB_GSO_FRAGLIST skb without GSO_BY_FRAGS is expected to have all
segments except the last to be gso_size long. If this does not hold,
the skb has been modified and the fraglist gso integrity is lost. Drop
the packet, as it cannot be segmented correctly by skb_segment_list.

The skb could be salvaged, though, right? By linearizing, dropping
the SKB_GSO_FRAGLIST bit and entering the normal skb_segment path
rather than the skb_segment_list path.

That choice is currently made in the protocol caller,
__udp_gso_segment. It's not trivial to add such a backup path here.
So let's add this backstop against kernel crashes.

> 
> If the gso_size does not match skb_headlen(),
> it means to be pulled part of or the entire
> fraglsit. It has been messed with and we return

fraglist

> error to free this skb.
> 
> 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 | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index b99127712e67..750fbb51b99f 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4493,6 +4493,7 @@ struct sk_buff *skb_segment_list(struct sk_buff
> *skb,
>  	unsigned int tnl_hlen = skb_tnl_header_len(skb);
>  	unsigned int delta_truesize = 0;
>  	unsigned int delta_len = 0;
> +	unsigned int mss = skb_shinfo(skb)->gso_size;

Reverse christmas tree

>  	struct sk_buff *tail = NULL;
>  	struct sk_buff *nskb, *tmp;
>  	int len_diff, err;
> @@ -4504,6 +4505,9 @@ struct sk_buff *skb_segment_list(struct sk_buff
> *skb,
>  	if (err)
>  		goto err_linearize;
>  
> +	if (mss != GSO_BY_FRAGS && mss != skb_headlen(skb))
> +		return ERR_PTR(-EFAULT);
> +

Do this precondition integrity check before the skb_unclone path?

>  	skb_shinfo(skb)->frag_list = NULL;
>  
>  	while (list_skb) {
> -- 
> 2.18.0