af_unix: Allow passing cred for embryo without SO_PASSCRED/SO_PASSPIDFD.

Before the cited commit, the kernel unconditionally embedded SCM
credentials to skb for embryo sockets even when both the sender
and listener disabled SO_PASSCRED and SO_PASSPIDFD.

Now, the credentials are added to skb only when configured by the
sender or the listener.

However, as reported in the link below, it caused a regression for
some programs that assume credentials are included in every skb,
but sometimes not now.

The only problematic scenario would be that a socket starts listening
before setting the option.  Then, there will be 2 types of non-small
race window, where a client can send skb without credentials, which
the peer receives as an "invalid" message (and aborts the connection
it seems ?):

  Client                    Server
  ------                    ------
                            s1.listen()  <-- No SO_PASS{CRED,PIDFD}
  s2.connect()
  s2.send()  <-- w/o cred
                            s1.setsockopt(SO_PASS{CRED,PIDFD})
  s2.send()  <-- w/  cred

or

  Client                    Server
  ------                    ------
                            s1.listen()  <-- No SO_PASS{CRED,PIDFD}
  s2.connect()
  s2.send()  <-- w/o cred
                            s3, _ = s1.accept()  <-- Inherit cred options
  s2.send()  <-- w/o cred                            but not set yet

                            s3.setsockopt(SO_PASS{CRED,PIDFD})
  s2.send()  <-- w/  cred

It's unfortunate that buggy programs depend on the behaviour,
but let's restore the previous behaviour.

Fixes: 3f84d577b79d ("af_unix: Inherit sk_flags at connect().")
Reported-by: Jacek Łuczak <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Tested-by: Christian Heusel <[email protected]>
Tested-by: André Almeida <[email protected]>
Tested-by: Jacek Łuczak <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

0 files changed, 0 insertions, 0 deletions