netdev - Re: [PATCH] netfilter: add overflow checks in xt

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Date:   Mon, 4 Dec 2017 11:36:03 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Willem de Bruijn <willemb@...gle.com>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        coreteam@...filter.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

On Fri, Dec 01, 2017 at 01:46:07AM +0100, Jann Horn wrote:
> Check whether inputs from userspace are too long (explicit length field too
> big or string not null-terminated) to avoid out-of-bounds reads.
> 
> As far as I can tell, this can at worst lead to very limited kernel heap
> memory disclosure or oopses.
> 
> This bug can be triggered by an unprivileged user even if the xt_bpf module
> is not loaded: iptables is available in network namespaces, and the xt_bpf
> module can be autoloaded.
> 
> Triggering the bug with a classic BPF filter with fake length 0x1000 causes
> the following KASAN report:
> 
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in bpf_prog_create+0x84/0xf0
> Read of size 32768 at addr ffff8801eff2c494 by task test/4627
> 
> CPU: 0 PID: 4627 Comm: test Not tainted 4.15.0-rc1+ #1
> [...]
> Call Trace:
>  dump_stack+0x5c/0x85
>  print_address_description+0x6a/0x260
>  kasan_report+0x254/0x370
>  ? bpf_prog_create+0x84/0xf0
>  memcpy+0x1f/0x50
>  bpf_prog_create+0x84/0xf0
>  bpf_mt_check+0x90/0xd6 [xt_bpf]
> [...]
> Allocated by task 4627:
>  kasan_kmalloc+0xa0/0xd0
>  __kmalloc_node+0x47/0x60
>  xt_alloc_table_info+0x41/0x70 [x_tables]
> [...]
> The buggy address belongs to the object at ffff8801eff2c3c0
>                 which belongs to the cache kmalloc-2048 of size 2048
> The buggy address is located 212 bytes inside of
>                 2048-byte region [ffff8801eff2c3c0, ffff8801eff2cbc0)
> [...]
> ==================================================================

Applied, thanks.