netdev - Re: [PATCH net-next v4 02/11] net-timestamp: prepare for bpf prog use

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Message-ID: <c1701350-236d-4a9e-9c53-4badc0738309@linux.dev>
Date: Fri, 13 Dec 2024 14:26:26 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com,
 willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v4 02/11] net-timestamp: prepare for bpf prog use

On 12/13/24 6:42 AM, Jason Xing wrote:
>>>> I just noticed a trickier one, sockops bpf prog can write to sk->sk_txhash. The
>>>> same should go for reading from sk. Also, sockops prog assumes a fullsock sk is
>>>> a tcp_sock which also won't work for the udp case. A quick thought is to do
>>>> something similar to is_fullsock. May be repurpose the is_fullsock somehow or a
>>>> new u8 is needed. Take a look at SOCK_OPS_{GET,SET}_FIELD. It avoids
>>>> writing/reading the sk when is_fullsock is false.

May be this message buried in the earlier reply or some piece was not clear, so 
worth to highlight here.

Take a look at how is_fullsock is used in SOCK_OPS_{GET,SET}_FIELD. I think a 
similar idea can be borrowed here.

>>>
>>> Do you mean that if we introduce a new field, then bpf prog can
>>> read/write the socket?
>>
>> The same goes for writing the sk, e.g. writing the sk->sk_txhash. It needs the
>> sk_lock held. Reading may be ok-ish. The bpf prog can read it anyway by
>> bpf_probe_read...etc.
>>
>> When adding udp timestamp callback later, it needs to stop reading the tcp_sock
>> through skops from the udp callback for sure. Do take a look at
>> SOCK_OPS_GET_TCP_SOCK_FIELD. I think we need to ensure the udp timestamp
>> callback won't break here before moving forward.
> 
> Agreed. Removing the "sock_ops.sk = sk;" is simple, but I still want
> the bpf prog to be able to read some fields from the socket under
> those new callbacks.

No need to remove "sock_ops.sk = sk;". Try to borrow the is_fullsock idea.

Overall, the new timestamp callback breaks assumptions like, sk_lock is held and 
is_fullsock must be a tcp_sock. This needs to be audited. In particular, please 
check sock_ops_func_proto() for all accessible bpf helpers. Also check the 
sock_ops_is_valid_access() and sock_ops_convert_ctx_access() for directly 
accessible fields without the helpers. In particular, the BPF_WRITE (able) 
fields and the tcp_sock fields.