Subject: Re: [Patch bpf] tcp_bpf: fix return value of tcp_bpf_sendmsg()

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 29 Aug 2024 11:36:41 -0700

On 8/22/24 1:45 PM, John Fastabend wrote:
> Cong Wang wrote:
>> On Wed, Aug 21, 2024 at 03:55:33PM +0100, Simon Horman wrote:
>>> On Tue, Aug 20, 2024 at 08:07:44PM -0700, Cong Wang wrote:
>>>> From: Cong Wang <[email-masked]>
>>>>
>>>> When we cork messages in psock->cork, the last message triggers the
>>>> flushing will result in sending a sk_msg larger than the current
>>>> message size. In this case, in tcp_bpf_send_verdict(), 'copied' becomes
>>>> negative at least in the following case:
>>>>
>>>> 468         case __SK_DROP:
>>>> 469         default:
>>>> 470                 sk_msg_free_partial(sk, msg, tosend);
>>>> 471                 sk_msg_apply_bytes(psock, tosend);
>>>> 472                 *copied -= (tosend + delta); // <==== HERE
>>>> 473                 return -EACCES;
>>>>
>>>> Therefore, it could lead to the following BUG with a proper value of
>>>> 'copied' (thanks to syzbot). We should not use negative 'copied' as a
>>>> return value here.
>>>>
>>>>    ------------[ cut here ]------------
>>>>    kernel BUG at net/socket.c:733!
>>>>    Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
>>>>    Modules linked in:
>>>>    CPU: 0 UID: 0 PID: 3265 Comm: syz-executor510 Not tainted 6.11.0-rc3-syzkaller-00060-gd07b43284ab3 #0
>>>>    Hardware name: linux,dummy-virt (DT)
>>>>    pstate: 61400009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
>>>>    pc : sock_sendmsg_nosec net/socket.c:733 [inline]
>>>>    pc : sock_sendmsg_nosec net/socket.c:728 [inline]
>>>>    pc : __sock_sendmsg+0x5c/0x60 net/socket.c:745
>>>>    lr : sock_sendmsg_nosec net/socket.c:730 [inline]
>>>>    lr : __sock_sendmsg+0x54/0x60 net/socket.c:745
>>>>    sp : ffff800088ea3b30
>>>>    x29: ffff800088ea3b30 x28: fbf00000062bc900 x27: 0000000000000000
>>>>    x26: ffff800088ea3bc0 x25: ffff800088ea3bc0 x24: 0000000000000000
>>>>    x23: f9f00000048dc000 x22: 0000000000000000 x21: ffff800088ea3d90
>>>>    x20: f9f00000048dc000 x19: ffff800088ea3d90 x18: 0000000000000001
>>>>    x17: 0000000000000000 x16: 0000000000000000 x15: 000000002002ffaf
>>>>    x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
>>>>    x11: 0000000000000000 x10: ffff8000815849c0 x9 : ffff8000815b49c0
>>>>    x8 : 0000000000000000 x7 : 000000000000003f x6 : 0000000000000000
>>>>    x5 : 00000000000007e0 x4 : fff07ffffd239000 x3 : fbf00000062bc900
>>>>    x2 : 0000000000000000 x1 : 0000000000000000 x0 : 00000000fffffdef
>>>>    Call trace:
>>>>     sock_sendmsg_nosec net/socket.c:733 [inline]
>>>>     __sock_sendmsg+0x5c/0x60 net/socket.c:745
>>>>     ____sys_sendmsg+0x274/0x2ac net/socket.c:2597
>>>>     ___sys_sendmsg+0xac/0x100 net/socket.c:2651
>>>>     __sys_sendmsg+0x84/0xe0 net/socket.c:2680
>>>>     __do_sys_sendmsg net/socket.c:2689 [inline]
>>>>     __se_sys_sendmsg net/socket.c:2687 [inline]
>>>>     __arm64_sys_sendmsg+0x24/0x30 net/socket.c:2687
>>>>     __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>>>>     invoke_syscall+0x48/0x110 arch/arm64/kernel/syscall.c:49
>>>>     el0_svc_common.constprop.0+0x40/0xe0 arch/arm64/kernel/syscall.c:132
>>>>     do_el0_svc+0x1c/0x28 arch/arm64/kernel/syscall.c:151
>>>>     el0_svc+0x34/0xec arch/arm64/kernel/entry-common.c:712
>>>>     el0t_64_sync_handler+0x100/0x12c arch/arm64/kernel/entry-common.c:730
>>>>     el0t_64_sync+0x19c/0x1a0 arch/arm64/kernel/entry.S:598
>>>>    Code: f9404463 d63f0060 3108441f 54fffe81 (d4210000)
>>>>    ---[ end trace 0000000000000000 ]---
>>>>
>>>> Fixes: 4f738adba30a ("bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data")
>>>> Reported-by: [email-masked]
>>>> Cc: John Fastabend <[email-masked]>
>>>> Cc: Jakub Sitnicki <[email-masked]>
>>>> Signed-off-by: Cong Wang <[email-masked]>
>>>> ---
>>>>   net/ipv4/tcp_bpf.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
>>>> index 53b0d62fd2c2..fe6178715ba0 100644
>>>> --- a/net/ipv4/tcp_bpf.c
>>>> +++ b/net/ipv4/tcp_bpf.c
>>>> @@ -577,7 +577,7 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>>>>   		err = sk_stream_error(sk, msg->msg_flags, err);
>>>>   	release_sock(sk);
>>>>   	sk_psock_put(sk, psock);
>>>> -	return copied ? copied : err;
>>>> +	return copied > 0 ? copied : err;
>>>
>>> Does it make more sense to make the condition err:
>>> is err 0 iif everything is ok? (completely untested!)
>>
>> Mind to elaborate?
>>
>>  From my point of view, 'copied' is to handle partial transmission, for
>> example:
>>
>> 0. User wants to send 2 * 1K bytes with sendmsg()
>> 1. Kernel already sent the first 1K successfully
>> 2. Kernel got some error when sending the 2nd 1K
>>
>> In this scenario, we should return 1K instead of the error to the caller to
>> indicate this partial transmission situation, otherwise we could not
>> distinguish it with a compete failure (that is, 0 byte sent).
> 
> Yep, if we don't return the positive value on partial send we will confuse
> apps and they will probably resent data.
> 
>  From my side this looks good.
> 
> Reviewed-by: John Fastabend <[email-masked]>

Acked-by: Martin KaFai Lau <[email-masked]>

Jakub, can you directly land it to the net tree?