IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[Jesper Juhl]

Hi,

In net/ipv4/ip_options.c::ip_options_compile() we have the following
code at the start of the function :


int ip_options_compile(struct ip_options * opt, struct sk_buff * skb)
{
        int l;
        unsigned char * iph;
        unsigned char * optptr;
        int optlen;
        unsigned char * pp_ptr = NULL;
        struct rtable *rt = skb ? (struct rtable*)skb->dst : NULL;

        if (!opt) {
                opt = &(IPCB(skb)->opt);
                iph = skb->nh.raw;
                opt->optlen = ((struct iphdr *)iph)->ihl*4 -
sizeof(struct iphdr);
                optptr = iph + sizeof(struct iphdr);
                opt->is_data = 0;
        } else {
                optptr = opt->is_data ? opt->__data : (unsigned
char*)&(skb->nh.iph[1]);
                iph = optptr - sizeof(struct iphdr);
        }

...

I don't see how that avoids blowing up if we get passed a NULL skb.

>From the line
    struct rtable *rt = skb ? (struct rtable*)skb->dst : NULL;
it is clear that we /may/ get passed a null skb.

Then a bit further down in the  if (!opt)  bit we dereference 'skb' :
    opt = &(IPCB(skb)->opt);
and we also may dereference it in the  else  part :
    optptr = opt->is_data ? opt->__data : (unsigned char*)&(skb->nh.iph[1]);

So if 'skb' is NULL, the only route I see that doesn't cause a NULL
pointer deref is if  (opt != NULL)  and at the same time
(opt->is_data != NULL)  .   Is that guaranteed in any way?  If now,
how come we don't blow up regularly?


-- 
Jesper Juhl <jesper.juhl@gmail.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
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Re: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[David Miller]

From: "Jesper Juhl" <jesper.juhl@gmail.com>
Date: Thu, 16 Nov 2006 23:34:14 +0100

> So if 'skb' is NULL, the only route I see that doesn't cause a NULL
> pointer deref is if  (opt != NULL)  and at the same time
> (opt->is_data != NULL)  .   Is that guaranteed in any way?

Yes, it is guarenteed, all callers  make sure these invariants
hold true.

I'm very happy to accept a patch which assert's this using BUG()
checks :-)

Re: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[Herbert Xu]

Jesper Juhl <jesper.juhl@gmail.com> wrote:
> 
> So if 'skb' is NULL, the only route I see that doesn't cause a NULL
> pointer deref is if  (opt != NULL)  and at the same time
> (opt->is_data != NULL)  .   Is that guaranteed in any way?  If now,
> how come we don't blow up regularly?

Yes that's how it's supposed to be used.  This function either constructs
an opts structure from a packet, or it verifies the validity of a suspect
opts structure (without a packet).  In the latter case, both opt and
opt->is_data should be non-zero.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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Re: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[Herbert Xu]

David Miller <davem@davemloft.net> wrote:
>
> I'm very happy to accept a patch which assert's this using BUG()
> checks :-)

A BUG() won't be necessary because the NULL pointer dereferences will
OOPS anyway.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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Re: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[Dmitry Torokhov]

On Thursday 16 November 2006 19:37, Herbert Xu wrote:
> David Miller <davem@davemloft.net> wrote:
> >
> > I'm very happy to accept a patch which assert's this using BUG()
> > checks :-)
> 
> A BUG() won't be necessary because the NULL pointer dereferences will
> OOPS anyway.
>

BUG()s there would be a mechanism to document invariants so next time
someone is looking at the code there are no questions.

-- 
Dmitry

Re: IPv4: ip_options_compile() how can we avoid blowing up on a NULL skb???

[Herbert Xu]

Dmitry Torokhov <dtor@insightbb.com> wrote:
> 
> BUG()s there would be a mechanism to document invariants so next time
> someone is looking at the code there are no questions.

Well if someone is documenting this then wouldn't a comment (or even a
kerneldoc style block) be better?

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt