netdev - Re: [PATCH net] ipv4: Fix use-after-free when flushing FIB tables

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Message-Id: <20171219.113221.332118764426444996.davem@davemloft.net>
Date:   Tue, 19 Dec 2017 11:32:21 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     idosch@...lanox.com
Cc:     netdev@...r.kernel.org, alexander.h.duyck@...el.com,
        fengguang.wu@...el.com, dsahern@...il.com, mlxsw@...lanox.com
Subject: Re: [PATCH net] ipv4: Fix use-after-free when flushing FIB tables

From: Ido Schimmel <idosch@...lanox.com>
Date: Mon, 18 Dec 2017 10:13:20 +0200

> Since commit 0ddcf43d5d4a ("ipv4: FIB Local/MAIN table collapse") the
> local table uses the same trie allocated for the main table when custom
> rules are not in use.
> 
> When a net namespace is dismantled, the main table is flushed and freed
> (via an RCU callback) before the local table. In case the callback is
> invoked before the local table is iterated, a use-after-free can occur.
> 
> Fix this by iterating over the FIB tables in reverse order, so that the
> main table is always freed after the local table.
> 
> Fixes: 0ddcf43d5d4a ("ipv4: FIB Local/MAIN table collapse")
> Signed-off-by: Ido Schimmel <idosch@...lanox.com>
> Reported-by: Fengguang Wu <fengguang.wu@...el.com>

This is really too clever of a fix I think :-)

I would prefer if we fixed things more explicitly.

In struct fib_table you can add a "data_ref" integer.  Any pointer
reference created to fib_table->__data increases this counter.  It is
always done inside of RTNL locking, so should be doable without
atomics or extra locking.

For a non-aliased fib_table we go:

	if (!--fib_table->data_ref)
		kfree(fib_table);

And for aliased ones we do something like:

	if (fib_table->tb_data != fib_table->__data) {
		void *data = fib_table->fb_data;
		struct fib_table *alias;

		alias = container_of(data, struct fib_table, __data[0]);
		if (!--alias->data_ref)
			kfree(alias);
		kfree(fib_table);
	}

Something like that.