Subject: Re: [PATCH] netfilter: nfnetlink: fix uninitialized local variable

From: Breno Leitao <[email-masked]>
Date: Thu, 15 Aug 2024 02:55:19 -0700

Hello Pablo,

On Thu, Aug 15, 2024 at 11:32:51AM +0200, Pablo Neira Ayuso wrote:
> On Thu, Aug 15, 2024 at 02:04:04AM -0700, Breno Leitao wrote:
> > On Thu, Aug 15, 2024 at 04:27:33PM +0800, icejl wrote:
> > > In the nfnetlink_rcv_batch function, an uninitialized local variable
> > > extack is used, which results in using random stack data as a pointer.
> > > This pointer is then used to access the data it points to and return
> > > it as the request status, leading to an information leak. If the stack
> > > data happens to be an invalid pointer, it can cause a pointer access
> > > exception, triggering a kernel crash.
> > > 
> > > Signed-off-by: icejl <[email-masked]>
> > > ---
> > >  net/netfilter/nfnetlink.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
> > > index 4abf660c7baf..b29b281f4b2c 100644
> > > --- a/net/netfilter/nfnetlink.c
> > > +++ b/net/netfilter/nfnetlink.c
> > > @@ -427,6 +427,7 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
> > >  
> > >  	nfnl_unlock(subsys_id);
> > >  
> > > +	memset(&extack, 0, sizeof(extack));
> > >  	if (nlh->nlmsg_flags & NLM_F_ACK)
> > >  		nfnl_err_add(&err_list, nlh, 0, &extack);
> > 
> > There is a memset later in that function , inside the 
> > `while (skb->len >= nlmsg_total_size(0))` loop. Should that one be
> > removed?
> 
> no, the batch contains a series of netlink message, each of them needs
> a fresh extack area which is zeroed.

Sorry, this is a bit unclear to me. This is the code I see in
netnext/main:

	memset(&extack, 0, sizeof(extack));   // YOUR CHANGE

        if (nlh->nlmsg_flags & NLM_F_ACK)
                nfnl_err_add(&err_list, nlh, 0, &extack);

        while (skb->len >= nlmsg_total_size(0)) {
                int msglen, type;

                if (fatal_signal_pending(current)) {
                        nfnl_err_reset(&err_list);
                        err = -EINTR;
                        status = NFNL_BATCH_FAILURE;
                        goto done;
                }

->              memset(&extack, 0, sizeof(extack));

nfnl_err_add() does not change extack. Tht said, the second memset (last
line in the snippet above), seems useless, doesn't it?

Thanks for the quick reply,
--breno