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

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Message-ID: <CAF=yD-JRfKGjvPq4cMQADbNJc2C+0FBxAO2v=xNHKc7y-O=rJQ@mail.gmail.com>
Date:   Thu, 30 Nov 2017 23:04:43 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Willem de Bruijn <willemb@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        coreteam@...filter.org,
        netfilter-devel <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn <jannh@...gle.com> 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)
> [...]
> ==================================================================
>
> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
>  net/netfilter/xt_bpf.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/net/netfilter/xt_bpf.c b/net/netfilter/xt_bpf.c
> index 041da0d9c06f..1f7fbd3c7e5a 100644
> --- a/net/netfilter/xt_bpf.c
> +++ b/net/netfilter/xt_bpf.c
> @@ -27,6 +27,9 @@ static int __bpf_mt_check_bytecode(struct sock_filter *insns, __u16 len,
>  {
>         struct sock_fprog_kern program;
>
> +       if (len > XT_BPF_MAX_NUM_INSTR)
> +               return -EINVAL;
> +
>         program.len = len;
>         program.filter = insns;

Next, this calls bpf_prog_create, which calls bpf_check_basics_ok to verify len.

> @@ -55,6 +58,9 @@ static int __bpf_mt_check_path(const char *path, struct bpf_prog **ret)
>         mm_segment_t oldfs = get_fs();
>         int retval, fd;
>
> +       if (strnlen(path, XT_BPF_PATH_MAX) == XT_BPF_PATH_MAX)
> +               return -EINVAL;
> +

Good catch. It looks like this code needs a more thorough revision.

https://lkml.kernel.org/r/<20171201034859.GN21978@...IV.linux.org.uk>