netdev - RE: [PATCH net-next] skbuff: revert "skbuff: remove some unnecessary operation in skb_segment

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Date:   Thu, 15 Apr 2021 13:10:32 +0900
From:   "Dongseok Yi" <dseok.yi@...sung.com>
To:     "'Yunsheng Lin'" <linyunsheng@...wei.com>,
        "'Paolo Abeni'" <pabeni@...hat.com>, <netdev@...r.kernel.org>
Cc:     "'Willem de Bruijn'" <willemb@...gle.com>
Subject: RE: [PATCH net-next] skbuff: revert
 "skbuff: remove some unnecessary operation in skb_segment_list()"

On Thu, Apr 15, 2021 at 10:23:17AM +0800, Yunsheng Lin wrote:
> On 2021/4/14 18:48, Paolo Abeni wrote:
> > the commit 1ddc3229ad3c ("skbuff: remove some unnecessary operation
> > in skb_segment_list()") introduces an issue very similar to the
> > one already fixed by commit 53475c5dd856 ("net: fix use-after-free when
> > UDP GRO with shared fraglist").
> >
> > If the GSO skb goes though skb_clone() and pskb_expand_head() before
> > entering skb_segment_list(), the latter  will unshare the frag_list
> > skbs and will release the old list. With the reverted commit in place,
> > when skb_segment_list() completes, skb->next points to the just
> > released list, and later on the kernel will hit UaF.
> 
> In that case, is "nskb->next = list_skb" needed before jumpping to
> error when __skb_linearize() fails? As there is "nskb->next = list_skb"
> before jumpping to error handling when skb_clone() fails.
> 
> The inconsistency above is the reason I sent the reverted patch:)

Yes, It is needed. skb_clone clears the next pointer.

> 
> >
> > Note that since commit e0e3070a9bc9 ("udp: properly complete L4 GRO
> > over UDP tunnel packet") the critical scenario can be reproduced also
> > receiving UDP over vxlan traffic with:
> >
> > NIC (NETIF_F_GRO_FRAGLIST enabled) -> vxlan -> UDP sink
> >
> > Attaching a packet socket to the NIC will cause skb_clone() and the
> > tunnel decapsulation will call pskb_expand_head().
> >
> > Fixes: 1ddc3229ad3c ("skbuff: remove some unnecessary operation in skb_segment_list()")
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> >  net/core/skbuff.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 3ad9e8425ab2..14010c0eec48 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -3773,13 +3773,13 @@ 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;
> > +	struct sk_buff *tail = NULL;
> >  	struct sk_buff *nskb, *tmp;
> >  	int err;
> >
> >  	skb_push(skb, -skb_network_offset(skb) + offset);
> >
> >  	skb_shinfo(skb)->frag_list = NULL;
> > -	skb->next = list_skb;
> >
> >  	do {
> >  		nskb = list_skb;
> > @@ -3797,8 +3797,17 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> >  			}
> >  		}
> >
> > -		if (unlikely(err))
> > +		if (!tail)
> > +			skb->next = nskb;
> > +		else
> > +			tail->next = nskb;
> > +
> > +		if (unlikely(err)) {
> > +			nskb->next = list_skb;
> >  			goto err_linearize;
> > +		}
> > +
> > +		tail = nskb;
> >
> >  		delta_len += nskb->len;
> >  		delta_truesize += nskb->truesize;
> > @@ -3825,7 +3834,7 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> >
> >  	skb_gso_reset(skb);
> >
> > -	skb->prev = nskb;
> > +	skb->prev = tail;
> >
> >  	if (skb_needs_linearize(skb, features) &&
> >  	    __skb_linearize(skb))
> >