Re: slab corruption with current -git

Linus Torvalds [off-list ref] wrote:

On Wed, Oct 12, 2016 at 11:27 PM, Markus Trippelsdorf
[off-list ref] wrote:

quoted

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.