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

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Date: Fri, 1 Dec 2023 13:14:53 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <john.fastabend@...il.com>
CC: <bpf@...r.kernel.org>, <edumazet@...gle.com>, <jakub@...udflare.com>,
	<martin.lau@...nel.org>, <netdev@...r.kernel.org>, <kuniyu@...zon.com>
Subject: [PATCH bpf v2 1/2] bpf: syzkaller found null ptr deref in unix_bpf proto add

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 ?