Re: UNIX(7)

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Alejandro Colomar <alx@kernel.org>
Date: Fri, 1 Dec 2023 13:54:39 +0100
> Hello Alexey,
> 
> On Fri, Dec 01, 2023 at 01:16:27PM +0100, Alexey Tikhonov wrote:
> > Hello.
> > 
> > There is a discrepancy between the man page description of
> > 'SO_PEERCRED' and real behavior.
> > 
> > `man 7 unix` states:
> > ```
> >        SO_PEERCRED
> >               This read-only socket option returns the credentials of
> >               the peer process connected to this socket.  The returned
> >               credentials are those that were in effect at the time of
> >               the call to connect(2) or socketpair(2).
> > ```
> > 
> > This doesn't match real behavior in following situation (just an example):
> >  - process starts with uid=0, gid=0
> >  - process creates UNIX socket, binds it, listens on it
> >  - process changes to uid=uid1, git=gid1 (using `setresuid()`, `setresgid()`)
> >  - another process connects to the listening socket and requests
> > peer's credentials using `getsockopt(... SOL_SOCKET, SO_PEERCRED ...)`
> > 
> > According to the man page: SO_PEERCRED should report (uid1, gid1),
> > because peer process was running under (uid1, gid1) "at the time of
> > the call to connect(2)"
> > In reality SO_PEERCRED reports (0, 0)
> > Reproducing code is available in
> > https://bugzilla.redhat.com/show_bug.cgi?id=2247682
> > 
> > I'm not entirely sure if this is a real bug or rather a  poor
> > description in the man page, but I tend to think that it's the latter.

When calling getsockopt(), we cannot know dynamically who the peer's
owner is.  So, we just initialise the cred when we know the owner,
and it's the caller of listen(), connect(), and socketpair().

In your case, the listener's cred is initialised with the caller's
cred during the first liten().

  listener's peer_cred = get_cred(rcu_dereference_protected(current->cred, 1))

And connect() will initialise two creds as follows:

  connect()er's peer_cred = listener's peer_cred
  new socket's peer_cred = get_cred(rcu_dereference_protected(current->cred, 1))

If you call listen() again after setresuid() and before connect(),
you can update the listener's cred and get the new IDs at the final
getsockopt().