netdev - Re: [PATCH v2 net-next 7/9] af_unix: Inherit sk

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Message-ID: <20250513032012.95677-1-kuniyu@amazon.com>
Date: Mon, 12 May 2025 20:20:07 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <willemdebruijn.kernel@...il.com>
CC: <brauner@...nel.org>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<horms@...nel.org>, <kuba@...nel.org>, <kuni1840@...il.com>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<willemb@...gle.com>
Subject: Re: [PATCH v2 net-next 7/9] af_unix: Inherit sk_flags at connect().

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Mon, 12 May 2025 22:48:09 -0400
> Kuniyuki Iwashima wrote:
> > From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> > Date: Mon, 12 May 2025 15:38:13 -0400
> > > Kuniyuki Iwashima wrote:
> > > > For SOCK_STREAM embryo sockets, the SO_PASS{CRED,PIDFD,SEC} options
> > > > are inherited from the parent listen()ing socket.
> > > > 
> > > > Currently, this inheritance happens at accept(), because these
> > > > attributes were stored in sk->sk_socket->flags and the struct socket
> > > > is not allocated until accept().
> > > > 
> > > > This leads to unintentional behaviour.
> > > > 
> > > > When a peer sends data to an embryo socket in the accept() queue,
> > > > unix_maybe_add_creds() embeds credentials into the skb, even if
> > > > neither the peer nor the listener has enabled these options.
> > > > 
> > > > If the option is enabled, the embryo socket receives the ancillary
> > > > data after accept().  If not, the data is silently discarded.
> > > > 
> > > > This conservative approach works for SO_PASS{CRED,PIDFD,SEC}, but not
> > > > for SO_PASSRIGHTS; once an SCM_RIGHTS with a hung file descriptor is
> > > > sent, it’s game over.
> > > 
> > > Should this be a fix to net then?
> > 
> > Regarding SO_PASS{CRED,PIDFD,SEC}, this patch is a small optimisation
> > to save unnecessary get_pid() etc, like 16e572626961
> > 
> > And, SO_PASSRIGHTS is not yet added here, so this is not a fix.
> 
> Ack, thanks.
>  
> > Maybe I should have clarified like "this works but would not for SO_PASSRIGHTS".
> > 
> > 
> > > 
> > > It depends on the move of this one bit from socket to sock. So is not
> > > a stand-alone patch. But does not need all of the previous cleanup
> > > patches if needs to be backportable.
> > > 
> > > > 
> > > > To avoid this, we will need to preserve SOCK_PASSRIGHTS even on embryo
> > > > sockets.
> > > > 
> > > > A recent change made it possible to access the parent's flags in
> > > > sendmsg() via unix_sk(other)->listener->sk->sk_socket->flags, but
> > > > this introduces an unnecessary condition that is irrelevant for
> > > > most sockets, accept()ed sockets and clients.
> > > 
> > > What is this condition and how is it irrelevant? A constraint on the
> > > kernel having the recent change? I.e., not backportable?
> > 
> > Commit aed6ecef55d7 ("af_unix: Save listener for embryo socket.") is
> > added for a new GC but is a standalone patch.
> > 
> > If we want to use the listener's flags, the condition will be like...
> > 
> > 	if (UNIXCB(skb).fp &&
> > 	    ((other->sk_socket && other->sk_socket->sk_flags & SOCK_PASSRIGHTS) ||
> > 	     (!other->sk_socket && unix_sk(other)->listener->sk->sk_socket->sk_flags && SOCK_PASSRIGHTS)))
> 
> No need to change as this stays as net-next.
> 
> Might we helpful to replace "a recent commit" with the full commit reference.

Will do.

Thanks!