Re: [PATCH] flow_dissector: prevent NULL pointer dereference in __skb_flow_dissect

[Anastasia Belova]

21/03/24 13:57, Jiri Pirko пишет:
> Thu, Mar 21, 2024 at 10:36:53AM CET, abelova@xxxxxxxxxxxxx wrote:
> > 20/03/24 16:38, Jiri Pirko пишет:
> > > Wed, Mar 20, 2024 at 01:56:35PM CET, abelova@xxxxxxxxxxxxx wrote:
> > > > skb is an optional parameter, so it may be NULL.
> > > > Add check defore dereference in eth_hdr.
> > > >
> > > > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > > Either drop this line which provides no value, or attach a link to the
> > > actual report.
> > It is an established practice for our project. You can find 700+ applied
> > patches with similar line:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?qt=grep&q=linuxtesting.org
> Okay. So would it be possible to attach a link to the actual report?

Unfortunately no as far as results of the SVACE static analysis tool are
not available publicly at the moment.


Also I agree that this is quite a minor fix, but I still insist
that it would be better to add a check.

> > > > Fixes: 67a900cc0436 ("flow_dissector: introduce support for Ethernet addresses")
> > > This looks incorrect. I believe that this is the offending commit:
> > > commit 690e36e726d00d2528bc569809048adf61550d80
> > > Author: David S. Miller <davem@xxxxxxxxxxxxx>
> > > Date:   Sat Aug 23 12:13:41 2014 -0700
> > >
> > >       net: Allow raw buffers to be passed into the flow dissector.
> > Got it.
> >
> > > > Signed-off-by: Anastasia Belova <abelova@xxxxxxxxxxxxx>
> > > > ---
> > > > net/core/flow_dissector.c | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > > > index 272f09251343..05db3a8aa771 100644
> > > > --- a/net/core/flow_dissector.c
> > > > +++ b/net/core/flow_dissector.c
> > > > @@ -1137,7 +1137,7 @@ bool __skb_flow_dissect(const struct net *net,
> > > > 		rcu_read_unlock();
> > > > 	}
> > > >
> > > > -	if (dissector_uses_key(flow_dissector,
> > > > +	if (skb && dissector_uses_key(flow_dissector,
> > > > 			       FLOW_DISSECTOR_KEY_ETH_ADDRS)) {
> > > > 		struct ethhdr *eth = eth_hdr(skb);
> > > > 		struct flow_dissector_key_eth_addrs *key_eth_addrs;
> > > Looks like FLOW_DISSECT_RET_OUT_BAD should be returned in case the
> > > FLOW_DISSECTOR_KEY_ETH_ADDRS are selected and there is no skb, no?
> > I agree, I'll send the second version.
> >
> > Anastasia Belova