Re: net: fix incorrect credentials passing

[David Miller <davem@davemloft.net>]

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 19 Apr 2013 18:32:32 -0700 (PDT)

> Commit 257b5358b32f ("scm: Capture the full credentials of the scm 
> sender") changed the credentials passing code to pass in the effective 
> uid/gid instead of the real uid/gid.

Yep, that's exactly what happened, via a very twisty and winding road.

The helper cred_to_ucred(), which was created by commit
3f551f9436c05a3b5eccdd6e94733df5bb98d2a5 ("sock: Introduce
cred_to_ucred") seemed to be aimed at providing the semantics expected
by SO_PEERCRED, which does in fact want to use the effective UIDs and
GIDs.

The SO_PEERCRED side of this can be seen more clearly in commit
109f6e39fa07c48f580125f531f46cb7c245b528 ("af_unix: Allow SO_PEERCRED
to work across namespaces.")

The error begins when we try to use that helper in the SCM credentials
code.

Initially, cred_to_ucred() is what was used by scm_set_cred().  Then
it got changed in dbe9a4173ea53b72b2c35d19f676a85b69f1c9fe ("scm:
Don't use struct ucred in NETLINK_CB and struct scm_cookie.") which
continues the error by propagating the effective UID/GID usage from
cred_to_ucred() into explicit accesses inside of scm_set_cred().

So, as stated, the error originates from trying to use this common
helper for both SO_PEERCERD and the SCM stuff, the latter of which
does not want to use effective UIDs and GIDs.

> This just undoes that (presumably unintentional) part of the commit.

Indeed, I'd say this was an oversight in semantic differences when
creating the cred_to_ucred() helper.

Eric B. please review and ACK this.

Thanks!