BUG: kernel NULL pointer dereference while copying sk_buff struct

[Alexander Kapshuk]

On Tue, May 2, 2023 at 3:26 PM Abdul Matin
<abdulmatincuetcse17 at gmail.com> wrote:
>
> Hi.
> I'm writing a netfilter module where I need to copy a sk_buff in a global variable that I use in another subsequent call. But I crashed the whole kernel. I've tried to add a code snippet to share with you how I'm doing it.
>
> here case1 is always true before case2 (i.e. 1st call of help -> case1 is true, 2nd call of help -> case2 true).
> So, in the 2nd call, case2 is true where we're using exp, ctinfoPrev, saddr_m which have been initialized before in case1.
>
> union nf_inet_addr *saddr_m;
> struct sk_buff* skbPrev;

This declares a pointer to struct sk_buff which is uninitialised and
most probably set to NULL by the compiler.
Where do you allocate memory for skbPrev?

> enum ip_conntrack_info ctinfoPrev;
> struct nf_conntrack_expect *exp;
>
> static int help(struct sk_buff *skb,
>     unsigned int protoff,
>     struct nf_conn *ct,
>     enum ip_conntrack_info ctinfo)
> {
>      switch (msgType) {
>     case case1:
>
>     ctinfoPrev = ctinfo;
>     memcpy((void *)skbPrev, (const void *)skb, sizeof(skb));

The NULL pointer dereference probably happens here, as memcpy attempts
 to copy data from skb to skbPrev, which is likely to be NULL.

>     skbPrev->next = (struct sk_buff*) kmalloc(sizeof(struct sk_buff), GFP_KERNEL);
>     skbPrev->prev = (struct sk_buff*) kmalloc(sizeof(struct sk_buff), GFP_KERNEL);
>     skbPrev->sk = (struct sock*) kmalloc(sizeof(struct sock), GFP_KERNEL);
>     memcpy((void *)(skbPrev->next), (const void *)skb->next, sizeof(skb->next));
>     memcpy((void *)(skbPrev->prev), (const void *)skb->prev, sizeof(skb->prev));
>    memcpy((void *)(skbPrev->sk), (const void *)skb->sk, sizeof(skb->sk));
>
>     unsigned int type = (dptr[0] << 8) | dptr[1]; // little endian
>     unsigned int length = (dptr[2] << 8) | dptr[3];
>     printk(KERN_INFO "type: %hu length: %hu", type, length);
>
>     unsigned int ip;
>     memcpy(&ip, dptr, 4);
>     ip = ntohl(ip) ^ MAGIC_COOKIE_VALUE_HOST;
>     exp = nf_ct_expect_alloc(ct);
>     if (exp == NULL) {
>         printk( KERN_INFO "cannot alloc expectation");
>         return NF_DROP;
>     }
>     tuple = &ct->tuplehash[IP_CT_DIR_REPLY].tuple;
>     nf_ct_expect_init(exp, NF_CT_EXPECT_CLASS_DEFAULT,
>     nf_ct_l3num(ct),
>     saddr_m, &tuple->dst.u3,
>     IPPROTO_UDP, NULL, &tuple->dst.u.udp.port);
>
>     pr_debug("expect: ");
>     nf_ct_dump_tuple(&exp->tuple);
>
>
>     break;
>    case case2:
>     printk(KERN_INFO "createpermission response\n");
>     nf_nat_tftp = rcu_dereference(nf_nat_tftp_hook);
>     if (nf_nat_tftp && ct->status & IPS_NAT_MASK)
>     ret= nf_nat_tftp(skbPrev, ctinfoPrev, exp);
>     else if (nf_ct_expect_related(exp, 0) != 0) {
>     printk( KERN_INFO "cannot add expectation");
>     nf_ct_helper_log(skb, ct, "cannot add expectation");
>     ret = NF_DROP;
>     }
>    nf_ct_expect_put(exp);
>    break;
>    }
>    return ret;
>   }
> I got this log before crash: 1,589,5743337757,-;BUG: kernel NULL pointer dereference, address: 0000000000000000
> 1,590,5743337860,-;#PF: supervisor read access in kernel mode
> 1,591,5743337880,-;#PF: error_code(0x0000) - not-present page
> 6,592,5743337900,-;PGD 0 P4D 0
> 4,593,5743337974,-;Oops: 0000 [#1] SMP PTI
>
> Is there anything wrong I am doing in copying and initializing?
>
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