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

From: Simon Horman <[email-masked]>
Date: Wed, 21 Aug 2024 15:55:33 +0100

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!)

	return err ? err : copied;

>  }
>  enum {
> -- 
> 2.34.1