netdev - Re: [PATCH bpf v2 1/2] bpf: syzkaller found null ptr deref in unix

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Date: Mon, 11 Dec 2023 15:56:04 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Cong Wang <xiyou.wangcong@...il.com>,
 John Fastabend <john.fastabend@...il.com>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, bpf@...r.kernel.org,
 edumazet@...gle.com, jakub@...udflare.com, martin.lau@...nel.org,
 netdev@...r.kernel.org, amery.hung@...edance.com
Subject: Re: [PATCH bpf v2 1/2] bpf: syzkaller found null ptr deref in
 unix_bpf proto add

On 12/8/23 5:19 AM, Cong Wang wrote:
> On Mon, Dec 04, 2023 at 01:40:40PM -0800, John Fastabend wrote:
>> Kuniyuki Iwashima wrote:
>>> From: John Fastabend <john.fastabend@...il.com>
>>> Date: Fri,  1 Dec 2023 10:01:38 -0800
>>>> I added logic to track the sock pair for stream_unix sockets so that we
>>>> ensure lifetime of the sock matches the time a sockmap could reference
>>>> the sock (see fixes tag). I forgot though that we allow af_unix unconnected
>>>> sockets into a sock{map|hash} map.
>>>>
>>>> This is problematic because previous fixed expected sk_pair() to exist
>>>> and did not NULL check it. Because unconnected sockets have a NULL
>>>> sk_pair this resulted in the NULL ptr dereference found by syzkaller.
>>>>
>>>> BUG: KASAN: null-ptr-deref in unix_stream_bpf_update_proto+0x72/0x430 net/unix/unix_bpf.c:171
>>>> Write of size 4 at addr 0000000000000080 by task syz-executor360/5073
>>>> Call Trace:
>>>>   <TASK>
>>>>   ...
>>>>   sock_hold include/net/sock.h:777 [inline]
>>>>   unix_stream_bpf_update_proto+0x72/0x430 net/unix/unix_bpf.c:171
>>>>   sock_map_init_proto net/core/sock_map.c:190 [inline]
>>>>   sock_map_link+0xb87/0x1100 net/core/sock_map.c:294
>>>>   sock_map_update_common+0xf6/0x870 net/core/sock_map.c:483
>>>>   sock_map_update_elem_sys+0x5b6/0x640 net/core/sock_map.c:577
>>>>   bpf_map_update_value+0x3af/0x820 kernel/bpf/syscall.c:167
>>>>
>>>> We considered just checking for the null ptr and skipping taking a ref
>>>> on the NULL peer sock. But, if the socket is then connected() after
>>>> being added to the sockmap we can cause the original issue again. So
>>>> instead this patch blocks adding af_unix sockets that are not in the
>>>> ESTABLISHED state.
>>>
>>> I'm not sure if someone has the unconnected stream socket use case
>>> though, can't we call additional sock_hold() in connect() by checking
>>> sk_prot under sk_callback_lock ?
>>
>> Could be done I guess yes. I'm not sure the utility of it though. I
>> thought above patch was the simplest solution and didn't require touching
>> main af_unix code. I don't actually use the sockmap with af_unix
>> sockets anywhere so maybe someone who is using this can comment if
>> unconnected is needed?
> 
> Our use case is also connected unix stream socket, as demonstrated in
> the selftest unix_redir_to_connected().

Great, is everyone good to move this fix forward then? Would be great if
this receives at least one ack if the latter is indeed the case.

Thanks,
Daniel