Re: [PATCH net v2] net: fix use-after-free when UDP GRO with shared fraglist

[Daniel Borkmann]

On 1/7/21 12:40 PM, Dongseok Yi wrote:
> On 2021-01-07 20:05, Daniel Borkmann wrote:
> > On 1/7/21 1:39 AM, Dongseok Yi wrote:
> > > skbs in fraglist could be shared by a BPF filter loaded at TC. It
> > > triggers skb_ensure_writable -> pskb_expand_head ->
> > > skb_clone_fraglist -> skb_get on each skb in the fraglist.
> > >
> > > While tcpdump, sk_receive_queue of PF_PACKET has the original fraglist.
> > > But the same fraglist is queued to PF_INET (or PF_INET6) as the fraglist
> > > chain made by skb_segment_list.
> > >
> > > If the new skb (not fraglist) is queued to one of the sk_receive_queue,
> > > multiple ptypes can see this. The skb could be released by ptypes and
> > > it causes use-after-free.
> > >
> > > [ 4443.426215] ------------[ cut here ]------------
> > > [ 4443.426222] refcount_t: underflow; use-after-free.
> > > [ 4443.426291] WARNING: CPU: 7 PID: 28161 at lib/refcount.c:190
> > > refcount_dec_and_test_checked+0xa4/0xc8
> > > [ 4443.426726] pstate: 60400005 (nZCv daif +PAN -UAO)
> > > [ 4443.426732] pc : refcount_dec_and_test_checked+0xa4/0xc8
> > > [ 4443.426737] lr : refcount_dec_and_test_checked+0xa0/0xc8
> > > [ 4443.426808] Call trace:
> > > [ 4443.426813]  refcount_dec_and_test_checked+0xa4/0xc8
> > > [ 4443.426823]  skb_release_data+0x144/0x264
> > > [ 4443.426828]  kfree_skb+0x58/0xc4
> > > [ 4443.426832]  skb_queue_purge+0x64/0x9c
> > > [ 4443.426844]  packet_set_ring+0x5f0/0x820
> > > [ 4443.426849]  packet_setsockopt+0x5a4/0xcd0
> > > [ 4443.426853]  __sys_setsockopt+0x188/0x278
> > > [ 4443.426858]  __arm64_sys_setsockopt+0x28/0x38
> > > [ 4443.426869]  el0_svc_common+0xf0/0x1d0
> > > [ 4443.426873]  el0_svc_handler+0x74/0x98
> > > [ 4443.426880]  el0_svc+0x8/0xc
> > >
> > > Fixes: 3a1296a38d0c (net: Support GRO/GSO fraglist chaining.)
> > > Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
> > > Acked-by: Willem de Bruijn <willemb@xxxxxxxxxx>
> > > ---
> > >    net/core/skbuff.c | 20 +++++++++++++++++++-
> > >    1 file changed, 19 insertions(+), 1 deletion(-)
> > >
> > > v2: Expand the commit message to clarify a BPF filter loaded
> > >
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index f62cae3..1dcbda8 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -3655,7 +3655,8 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> > >    	unsigned int delta_truesize = 0;
> > >    	unsigned int delta_len = 0;
> > >    	struct sk_buff *tail = NULL;
> > > -	struct sk_buff *nskb;
> > > +	struct sk_buff *nskb, *tmp;
> > > +	int err;
> > >
> > >    	skb_push(skb, -skb_network_offset(skb) + offset);
> > >
> > > @@ -3665,11 +3666,28 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
> > >    		nskb = list_skb;
> > >    		list_skb = list_skb->next;
> > >
> > > +		err = 0;
> > > +		if (skb_shared(nskb)) {
> > > +			tmp = skb_clone(nskb, GFP_ATOMIC);
> > > +			if (tmp) {
> > > +				kfree_skb(nskb);
> >
> > Should use consume_skb() to not trigger skb:kfree_skb tracepoint when looking
> > for drops in the stack.
>
> I will use to consume_skb() on the next version.
>
> > > +				nskb = tmp;
> > > +				err = skb_unclone(nskb, GFP_ATOMIC);
> >
> > Could you elaborate why you also need to unclone? This looks odd here. tc layer
> > (independent of BPF) from ingress & egress side generally assumes unshared skb,
> > so above clone + dropping ref of nskb looks okay to make the main skb struct private
> > for mangling attributes (e.g. mark) & should suffice. What is the exact purpose of
> > the additional skb_unclone() in this context?
>
> Willem de Bruijn said:
> udp_rcv_segment later converts the udp-gro-list skb to a list of
> regular packets to pass these one-by-one to udp_queue_rcv_one_skb.
> Now all the frags are fully fledged packets, with headers pushed
> before the payload.

Yes.

> PF_PACKET handles untouched fraglist. To modify the payload only
> for udp_rcv_segment, skb_unclone is necessary.

I don't parse this last sentence here, please elaborate in more detail on why
it is necessary.

For example, if tc layer would modify mark on the skb, then __copy_skb_header()
in skb_segment_list() will propagate it. If tc layer would modify payload, the
skb_ensure_writable() will take care of that internally and if needed pull in
parts from fraglist into linear section to make it private. The purpose of the
skb_clone() above iff shared is to make the struct itself private (to safely
modify its struct members). What am I missing?

> > > +			} else {
> > > +				err = -ENOMEM;
> > > +			}
> > > +		}
> > > +
> > >    		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;