linux-kernel - Re: [ 105/124] af_unix: dont send SCM_CREDENTIAL when dest socket is NULL

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Date:	Wed, 3 Apr 2013 12:01:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sven Joachim <svenjoac@....de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org, Ding Tianhong <dingtianhong@...wei.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [ 105/124] af_unix: dont send SCM_CREDENTIAL when dest socket is NULL

On Wed, Apr 3, 2013 at 11:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-04-03 at 10:58 -0700, Andy Lutomirski wrote:
>
>>
>> This sounds suspiciously like an SCM_CREDENTIALS bug triggered by a
>> race.  There's a fix (that needs both a new version from me and a review
>> by someone) here:
>>
>> http://www.spinics.net/lists/netdev/msg229948.html
>
> Hmm... this is not a stable candidate, IMHO.

Agreed.

>
> This has to be fixed (if needed) in a more easy way.
>
> What about this one liner ?
>
> CC Eric W. Biederman  as he wrote commit
> dbe9a4173ea53b72b2c3
> (scm: Don't use struct ucred in NETLINK_CB and struct scm_cookie.)
>
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 975cca0..42359d8 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -120,7 +120,7 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>                 return;
>         }
>
> -       if (test_bit(SOCK_PASSCRED, &sock->flags)) {
> +       if (test_bit(SOCK_PASSCRED, &sock->flags) && scm->creds.pid) {
>                 struct user_namespace *current_ns = current_user_ns();
>                 struct ucred ucreds = {
>                         .pid = scm->creds.pid,
>
>

That looks like it's correct.  If it gets applied, I'll respin my
patches on top of it.

(This approach may be a POSIX violation for all I know, and it's even
possible that some really fragile userspace breaks.  But I doubt it,
and anything that will break as a result is already operating in a
highly confused state; hence the original problem.)

--Andy
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