BUG: kernel NULL pointer dereference while copying sk_buff struct

[Alexander Kapshuk]

On Tue, May 2, 2023 at 6:35 PM Abdul Matin
<abdulmatincuetcse17 at gmail.com> wrote:
>
> I'm initializing the memory for skbPrev at module init function:
>
> static int __init nf_conntrack_my_mod_init(void)
> {
>     saddr_m = (union nf_inet_addr* ) kmalloc(sizeof(union nf_inet_addr), GFP_KERNEL);
>     skbPrev = (struct sk_buff*) kmalloc(sizeof(struct sk_buff), GFP_KERNEL);
>     if (saddr_m == NULL) {
>       printk(KERN_INFO "Can not allocate space for saddr\n");
>       return -ENOMEM;
>     }
>     if(skbPrev == NULL) {
>       printk(KERN_INFO "Can not allocate space for skbPrev\n");
>       return -ENOMEM;
>     }
>
> On Tue, May 2, 2023 at 8:23 PM Alexander Kapshuk <alexander.kapshuk at gmail.com> wrote:
>>
>> 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?
>> >
>> > _______________________________________________
>> > Kernelnewbies mailing list
>> > Kernelnewbies at kernelnewbies.org
>> > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

I see.
Without seeing the code in its entirety it's hard to tell which
pointer is being null-dereferenced.
Examine the OOPS output. The answer may very well be there.
Otherwise, you want to add some pr_info() or printk() calls around the
pointers being dereferenced in your code and see if any of those are
set to NULL.