On Tue, Jun 1, 2010 at 8:34 PM, jamal [off-list ref] wrote:

Hi Changli,

On Mon, 2010-05-31 at 10:24 +0800, Changli Gao wrote:

quoted

use skb_copy_bits() to dereference data safely

the original skb->data dereference isn't safe, as there isn't any skb->len or
skb_is_nonlinear() check.

I dont see any safety issue in current code in this respect. Do you have
a specific scenario where this would be unsafe? We inspect in 32 bit
chunks walking the packet data and stop when there is no more packet
data.

I added the following debug code into cls_u32.c

                for (i = n->sel.nkeys; i>0; i--, key++) {
+                       int off;
+
+                       off = key->off+(off2&key->offmask) + (ptr - skb->data);
+                       if (off + 4 > skb->len)
+                               printk("skb->len: %d, off: %d\n",
skb->len, off);

                        if
((*(__be32*)(ptr+key->off+(off2&key->offmask))^key->val)&key->mask) {
                                n = n->next;

@@ -179,8 +186,13 @@ check_terminal:

                ht = n->ht_down;
                sel = 0;
-               if (ht->divisor)
+               if (ht->divisor) {
+                       int off = ptr - skb->data + n->sel.hoff;
+                       if (off + 4 > skb->len)
+                               printk("skb->len: %d, off: %d\n", skb->len,
+                                       off);
                        sel =

ht->divisor&u32_hash_fold(*(__be32*)(ptr+n->sel.hoff),
&n->sel,n->fshift);
+               }

the tc filter command is:

tc filter add dev eth0 parent 8001:0 proto ip  u32 match u32
0x00000000 0x0000ffff at 40

Then I execute nc to connect to an UDP port:

localhost linux # nc -u 192.168.235.1 80

localhost linux #

just press enter.

I got these demsg:

[ 7525.330604] skb->len: 43, off: 54

we are trying to access the memory out of this skb.

quoted

skb_copy_bits() is used instead in this patch. And
when the skb isn't long enough, we terminate the function u32_classify()
immediately with -1.

That could be a very interesting optimization - but i see it as _very
hard_ to do with current u32 given it has branching and different
branches would have different lengths etc. You'd have to essentially do
some math in user space as to what min length would suffice given
a specified filter and pass that to the kernel.

It isn't an optimization, but an error exit. :)

-- 
Regards,
Changli Gao(xiaosuo@gmail.com)