Re: [PATCH] xfrm: xfrm_input: fix a possible memory leak in xfrm_input()

[Steffen Klassert <steffen.klassert@secunet.com>]

On Tue, May 31, 2022 at 10:12:05AM +0800, Hangyu Hua wrote:
> On 2022/5/30 18:37, Steffen Klassert wrote:
> > On Mon, May 30, 2022 at 06:20:46PM +0800, Hangyu Hua wrote:
> > > xfrm_input needs to handle skb internally. But skb is not freed When
> > > xo->flags & XFRM_GRO == 0 and decaps == 0.
> > > 
> > > Fixes: 7785bba299a8 ("esp: Add a software GRO codepath")
> > > Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
> > > ---
> > >   net/xfrm/xfrm_input.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
> > > index 144238a50f3d..6f9576352f30 100644
> > > --- a/net/xfrm/xfrm_input.c
> > > +++ b/net/xfrm/xfrm_input.c
> > > @@ -742,7 +742,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
> > >   			gro_cells_receive(&gro_cells, skb);
> > >   			return err;
> > >   		}
> > > -
> > > +		kfree_skb(skb);
> > >   		return err;
> > >   	}
> > 
> > Did you test this? The function behind the 'afinfo->the transport_finish()'
> > pointer handles this skb and frees it in that case.
> 
> int xfrm4_transport_finish(struct sk_buff *skb, int async)
> {
> 	struct xfrm_offload *xo = xfrm_offload(skb);
> 	struct iphdr *iph = ip_hdr(skb);
> 
> 	iph->protocol = XFRM_MODE_SKB_CB(skb)->protocol;
> 
> #ifndef CONFIG_NETFILTER
> 	if (!async)
> 		return -iph->protocol;		<--- [1]
> #endif
> ...
> 	NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING,
> 		dev_net(skb->dev), NULL, skb, skb->dev, NULL,
> 		xfrm4_rcv_encap_finish);	<--- [2]
> 	return 0;
> }
> 
> int xfrm6_transport_finish(struct sk_buff *skb, int async)
> {
> 	struct xfrm_offload *xo = xfrm_offload(skb);
> 	int nhlen = skb->data - skb_network_header(skb);
> 
> 	skb_network_header(skb)[IP6CB(skb)->nhoff] =
> 		XFRM_MODE_SKB_CB(skb)->protocol;
> 
> #ifndef CONFIG_NETFILTER
> 	if (!async)
> 		return 1;			<--- [3]
> #endif
> ...
> 	NF_HOOK(NFPROTO_IPV6, NF_INET_PRE_ROUTING,
> 		dev_net(skb->dev), NULL, skb, skb->dev, NULL,
> 		xfrm6_transport_finish2);
> 	return 0;				<--- [4]
> }
> 
> If transport_finish() return in [1] or [3], there will be a memory leak.

No, even in that case there is no memleak. Look for instance at the
IPv4 case, we return -iph->protocol here.
Then look at ip_protocol_deliver_rcu(). If the ipprot->handler (xfrm)
returns a negative value, this is interpreted as the protocol number
and the packet is resubmitted to the next protocol handler.

Please test your patches before you submit them in the future.