Subject: Re: slab corruption with current -git

From: Florian Westphal <[email-masked]>
Date: Thu, 13 Oct 2016 22:43:14 +0200

Linus Torvalds <[email-masked]> wrote:
> On Wed, Oct 12, 2016 at 11:27 PM, Markus Trippelsdorf
> <[email-masked]> wrote:
> >
> > Yeah.
> >
> > 105         entry->orig_ops = reg;
> > 106         entry->ops      = *reg;
> > 107         entry->next     = NULL;
> 
> So ipt_register_table() does:
> 
>         ret = nf_register_net_hooks(net, ops, hweight32(table->valid_hooks));
> 
> and then nf_register_net_hooks() just does
> 
>         for (i = 0; i < n; i++) {
>                 err = nf_register_net_hook(net, &reg[i]);
> 
> so if the *reg is uninitialized, it means that it's the 'ops[]' array
> that isn't actually really valid in "valid_hooks". Odd. They should
> all be initialized by xt_hook_ops_alloc(), no?

Its only partially initialized.  Looking at Markus' splat
its complaining about first 16 bytes (list_head), whose contents are indeed
undefined when it gets copied to entry->ops.

For the time being this seems like the most simple "fix", until we
disentangle the hook description (which should be const) from run-time
allocated data.

diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
index e0aa7c1d0224..fc4977456c30 100644
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -1513,7 +1513,7 @@ xt_hook_ops_alloc(const struct xt_table *table,
		nf_hookfn *fn)
        if (!num_hooks)
		return ERR_PTR(-EINVAL);
 
-       ops = kmalloc(sizeof(*ops) * num_hooks, GFP_KERNEL);
+       ops = kcalloc(num_hooks, sizeof(*ops), GFP_KERNEL);
        if (ops == NULL)
		return ERR_PTR(-ENOMEM);

I'll pass such a patch to Pablo.

> That said, xt_hook_ops_alloc() itself is odd. Lookie here, this is the
> loop that initializes things:
> 
>         for (i = 0, hooknum = 0; i < num_hooks && hook_mask != 0;
>              hook_mask >>= 1, ++hooknum) {
> 
> and it makes no sense to me how that tests *both* "i < num_hools" and
> "hook_mask != 0".

Right, one of these is enough.