Thanks for the tip! On Tue, May 2, 2023 at 10:18 PM Alexander Kapshuk < alexander.kaps...@gmail.com> wrote:
> On Tue, May 2, 2023 at 6:35 PM Abdul Matin > <abdulmatincuetcs...@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.kaps...@gmail.com> wrote: > >> > >> On Tue, May 2, 2023 at 3:26 PM Abdul Matin > >> <abdulmatincuetcs...@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@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. >
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