Commit 1a9a0592 authored Dec 02, 2016 by Alexey Dobriyan Committed by David S. Miller Dec 03, 2016

netns: publish net_generic correctly



Publishing net_generic pointer is done with silly mistake: new array is
published BEFORE setting freshly acquired pernet subsystem pointer.

	memcpy
	rcu_assign_pointer
	kfree_rcu
	ng->ptr[id - 1] = data;

This bug was introduced with commit dec827d1
("[NETNS]: The generic per-net pointers.") in the glorious days of
chopping networking stack into containers proper 8.5 years ago (whee...)

How it didn't trigger for so long?
Well, you need quite specific set of conditions:

*) race window opens once per pernet subsystem addition
   (read: modprobe or boot)

*) not every pernet subsystem is eligible (need ->id and ->size)

*) not every pernet subsystem is vulnerable (need incorrect or absense
   of ordering of register_pernet_sybsys() and actually using net_generic())

*) to hide the bug even more, default is to preallocate 13 pointers which
   is actually quite a lot. You need IPv6, netfilter, bridging etc together
   loaded to trigger reallocation in the first place. Trimmed down
   config are OK.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 4f7df337

net/core/net_namespace.c

+5 −5

Original line number	Diff line number	Diff line
		@@ -64,9 +64,10 @@ static int net_assign_generic(struct net net, unsigned int id, void data)

		old_ng = rcu_dereference_protected(net->gen,
		lockdep_is_held(&net_mutex));
		ng = old_ng;
		if (old_ng->len >= id)
		goto assign;
		if (old_ng->len >= id) {
		old_ng->ptr[id - 1] = data;
		return 0;
		}

		ng = net_alloc_generic();
		if (ng == NULL)
		@@ -84,11 +85,10 @@ static int net_assign_generic(struct net net, unsigned int id, void data)
		*/

		memcpy(&ng->ptr, &old_ng->ptr, old_ng->len * sizeof(void*));
		ng->ptr[id - 1] = data;

		rcu_assign_pointer(net->gen, ng);
		kfree_rcu(old_ng, rcu);
		assign:
		ng->ptr[id - 1] = data;
		return 0;
		}

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