Thanks for the tip! On Tue, May 2, 2023 at 10:18 PM Alexander Kapshuk < alexander.kapshuk@gmail.com> wrote:
On Tue, May 2, 2023 at 6:35 PM Abdul Matin <abdulmatincuetcse17@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@gmail.com> wrote:
On Tue, May 2, 2023 at 3:26 PM Abdul Matin <abdulmatincuetcse17@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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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.