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

["Lena Wang (王娜)" <Lena.Wang@mediatek.com>]

On Thu, 2024-04-25 at 10:07 -0400, Willem de Bruijn wrote:
>  	 
> External email : Please do not click links or open attachments until
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>  
> > > >  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?
> > 
> > After return error, the skb will enter into kfree_skb, not
> consume_skb.
> > It may meet same crash problem which has been resolved by
> skb_unclone.
> > 
> > Or kfree_skb could well handle the cloned skb's release?
> 
> Since this is an error path it should reach kfree_skb rather than
> consume_skb.
> 
So we could keep the check after the skb_unclone path, right?

> > 
> > Other changes are updated as below:
> > 
> > From 301da5c9d65652bac6091d4cd64b751b3338f8bb Mon Sep 17 00:00:00
> 2001
> > From: Shiming Cheng <shiming.cheng@mediatek.com>
> > 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@mediatek.com>
> > Signed-off-by: Lena Wang <lena.wang@mediatek.com>
> > ---
> >  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 64d55392debbc90ef2e9c33441024d612075bdd7 Mon Sep 17 00:00:00
> 2001
> > From: Shiming Cheng <shiming.cheng@mediatek.com>
> > Date: Wed, 24 Apr 2024 14:43:45 +0800
> > Subject: [PATCH net] net: drop pulled SKB_GSO_FRAGLIST skb
> > 
> > A 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.
> 
> Drop the "though, right?"
> > 
> > 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 part of or the entire fraglist has been pulled.
> > It has been messed with and we should return error to
> > free this skb.
> 
> This paragraph is now duplicative. Drop.

OK, updated as below:

From 59d561adc13d52e3c225c6b8276f6a53324f7d56 Mon Sep 17 00:00:00 2001
From: Shiming Cheng <shiming.cheng@mediatek.com>
Date: Wed, 24 Apr 2024 14:43:45 +0800
Subject: [PATCH net] net: drop pulled SKB_GSO_FRAGLIST skb

A 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. 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.

Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
Signed-off-by: Shiming Cheng <shiming.cheng@mediatek.com>
Signed-off-by: Lena Wang <lena.wang@mediatek.com>
---
 net/core/skbuff.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b99127712e67..4777f5fea6c3 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4491,6 +4491,7 @@ struct sk_buff *skb_segment_list(struct sk_buff
*skb,
 {
 	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
 	unsigned int tnl_hlen = skb_tnl_header_len(skb);
+	unsigned int mss = skb_shinfo(skb)->gso_size;
 	unsigned int delta_truesize = 0;
 	unsigned int delta_len = 0;
 	struct sk_buff *tail = NULL;
@@ -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);
+
 	skb_shinfo(skb)->frag_list = NULL;
 
 	while (list_skb) {
-- 
2.18.0

> > 
> > Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> > Signed-off-by: Shiming Cheng <shiming.cheng@mediatek.com>
> > Signed-off-by: Lena Wang <lena.wang@mediatek.com>
> > ---
> >  net/core/skbuff.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index b99127712e67..4777f5fea6c3 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -4491,6 +4491,7 @@ struct sk_buff *skb_segment_list(struct
> sk_buff
> > *skb,
> >  {
> >  struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> >  unsigned int tnl_hlen = skb_tnl_header_len(skb);
> > +unsigned int mss = skb_shinfo(skb)->gso_size;
> >  unsigned int delta_truesize = 0;
> >  unsigned int delta_len = 0;
> >  struct sk_buff *tail = NULL;
> > @@ -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);
> > +
> >  skb_shinfo(skb)->frag_list = NULL;
> >  
> >  while (list_skb) {
> > -- 
> > 2.18.0
> 
>