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

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Date: Wed, 06 Dec 2023 10:47:42 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: bpf@...r.kernel.org, edumazet@...gle.com, martin.lau@...nel.org,
 netdev@...r.kernel.org, kuniyu@...zon.com
Subject: Re: [PATCH bpf v2 1/2] bpf: syzkaller found null ptr deref in
 unix_bpf proto add

On Mon, Dec 04, 2023 at 01:40 PM -08, 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?
>
> From rcu and locking side looks like holding sk_callback_lock would
> be sufficient. I was thinking it would require a rcu grace period
> or something but seems not.

I'd revist the option of grabbing an skpair ref in unix_stream_sendmsg.