Subject: Re: [Eulerkernel] [PATCH] af_unix: dont send SCM_CREDENTIAL when dest socket is NULL

From: dingtianhong <[email-masked]>
Date: Tue, 26 Mar 2013 19:35:18 +0800

On 2013/3/26 12:32, Eric Dumazet wrote:
> On Tue, 2013-03-26 at 11:08 +0800, dingtianhong wrote:
>> On 2013/3/25 22:04, Eric Dumazet wrote:
>>> On Mon, 2013-03-25 at 18:28 +0800, dingtianhong wrote:
>>>> SCM_SCREDENTIALS should apply to write() syscalls only either source or destination
>>>> socket asserted SOCK_PASSCRED. The original implememtation in maybe_add_creds is wrong,
>>>> and breaks several LSB testcases ( i.e. /tset/LSB.os/netowkr/recvfrom/T.recvfrom).
>>>>
>>>> Origionally-authored-by: Karel Srot <[email-masked]>
>>>> Signed-off-by: Ding Tianhong <[email-masked]>
>>>> ---
>>>>    net/unix/af_unix.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>>>> index 51be64f..99189fd 100644
>>>> --- a/net/unix/af_unix.c
>>>> +++ b/net/unix/af_unix.c
>>>> @@ -1413,8 +1413,8 @@ static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
>>>>           if (UNIXCB(skb).cred)
>>>>                   return;
>>>>           if (test_bit(SOCK_PASSCRED, &sock->flags) ||
>>>> -           !other->sk_socket ||
>>>> -           test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
>>>> +           (other->sk_socket &&
>>>> +           test_bit(SOCK_PASSCRED, &other->sk_socket->flags))) {
>>>>                   UNIXCB(skb).pid  = get_pid(task_tgid(current));
>>>>                   UNIXCB(skb).cred = get_current_cred();
>>>>           }
>>>
>>> I am not sure why adding credentials if other->sk_socket is NULL could
>>> break an application ?
>> The bugzilla has report the bug:https://lsbbugs.linuxfoundation.org/show_bug.cgi?id=3523
>>
> 
> OK
> 
>>>
>>> This was the case before commit introducing this code.
>>
>> The commit 16e5726269(af_unix: dont send SCM_CREDENTIALS by default) may  introducing the problem.
>>
> 
> So the problem is that two messages have different credentials,
> because other->sk_socket changed between first and second message.
> 
> and unix_stream_recvmsg() has the following check :
> 
>                 if (check_creds) {
>                         /* Never glue messages from different writers */
>                         if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
>                             (UNIXCB(skb).cred != siocb->scm->cred))
>                                 break;
>                 } else {
>                         /* Copy credentials */
>                         scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
>                         check_creds = 1;
>                 }
> 
> In the case the receiver doesnt care at all (using recvfrom(), not recvmsg()), 
> we probably should not even call scm_set_creds() and avoid extra refcounting.
> 
I think if not call scm_set_creds(), the credential would useles in recvmsg().
we could remove code:
		if (check_creds) {
                        /* Never glue messages from different writers */
                        if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
                            (UNIXCB(skb).cred != siocb->scm->cred))
                                break;
                } else {
                        /* Copy credentials */
                        scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
                        check_creds = 1;
                }
> .