[PATCH net-next v5 0/3] Add SCM_PIDFD and SO_PEERPIDFD

[Alexander Mikhalitsyn]

1. Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
but it contains pidfd instead of plain pid, which allows programmers not
to care about PID reuse problem.

2. Add SO_PEERPIDFD which allows to get pidfd of peer socket holder pidfd.
This thing is direct analog of SO_PEERCRED which allows to get plain PID.

3. Add SCM_PIDFD / SO_PEERPIDFD kselftest

Idea comes from UAPI kernel group:
https://uapi-group.org/kernel-features/

Big thanks to Christian Brauner and Lennart Poettering for productive
discussions about this and Luca Boccassi for testing and reviewing this.

=== Motivation behind this patchset

Eric Dumazet raised a question:
> It seems that we already can use pidfd_open() (since linux-5.3), and
> pass the resulting fd in af_unix SCM_RIGHTS message ?

Yes, it's possible, but it means that from the receiver side we need
to trust the sent pidfd (in SCM_RIGHTS),
or always use combination of SCM_RIGHTS+SCM_CREDENTIALS, then we can
extract pidfd from SCM_RIGHTS,
then acquire plain pid from pidfd and after compare it with the pid
from SCM_CREDENTIALS.

A few comments from other folks regarding this.

Christian Brauner wrote:

>Let me try and provide some of the missing background.

>There are a range of use-cases where we would like to authenticate a
>client through sockets without being susceptible to PID recycling
>attacks. Currently, we can't do this as the race isn't fully fixable.
>We can only apply mitigations.

>What this patchset will allows us to do is to get a pidfd without the
>client having to send us an fd explicitly via SCM_RIGHTS. As that's
>already possibly as you correctly point out.

>But for protocols like polkit this is quite important. Every message is
>standalone and we would need to force a complete protocol change where
>we would need to require that every client allocate and send a pidfd via
>SCM_RIGHTS. That would also mean patching through all polkit users.

>For something like systemd-journald where we provide logging facilities
>and want to add metadata to the log we would also immensely benefit from
>being able to get a receiver-side controlled pidfd.

>With the message type we envisioned we don't need to change the sender
>at all and can be safe against pid recycling.

>Link: https://gitlab.freedesktop.org/polkit/polkit/-/merge_requests/154
>Link: https://uapi-group.org/kernel-features

Lennart Poettering wrote:

>So yes, this is of course possible, but it would mean the pidfd would
>have to be transported as part of the user protocol, explicitly sent
>by the sender. (Moreover, the receiver after receiving the pidfd would
>then still have to somehow be able to prove that the pidfd it just
>received actually refers to the peer's process and not some random
>process. – this part is actually solvable in userspace, but ugly)

>The big thing is simply that we want that the pidfd is associated
>*implicity* with each AF_UNIX connection, not explicitly. A lot of
>userspace already relies on this, both in the authentication area
>(polkit) as well as in the logging area (systemd-journald). Right now
>using the PID field from SO_PEERCREDS/SCM_CREDENTIALS is racy though
>and very hard to get right. Making this available as pidfd too, would
>solve this raciness, without otherwise changing semantics of it all:
>receivers can still enable the creds stuff as they wish, and the data
>is then implicitly appended to the connections/datagrams the sender
>initiates.

>Or to turn this around: things like polkit are typically used to
>authenticate arbitrary dbus methods calls: some service implements a
>dbus method call, and when an unprivileged client then issues that
>call, it will take the client's info, go to polkit and ask it if this
>is ok. If we wanted to send the pidfd as part of the protocol we
>basically would have to extend every single method call to contain the
>client's pidfd along with it as an additional argument, which would be
>a massive undertaking: it would change the prototypes of basically
>*all* methods a service defines… And that's just ugly.

>Note that Alex' patch set doesn't expose anything that wasn't exposed
>before, or attach, propagate what wasn't before. All it does, is make
>the field already available anyway (the struct ucred .pid field)
>available also in a better way (as a pidfd), to solve a variety of
>races, with no effect on the protocol actually spoken within the
>AF_UNIX transport. It's a seamless improvement of the status quo.

===

Git tree:
https://github.com/mihalicyn/linux/tree/scm_pidfd

Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Leon Romanovsky <leon@xxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Cc: Lennart Poettering <mzxreary@xxxxxxxxxxx>
Cc: Luca Boccassi <bluca@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>

Tested-by: Luca Boccassi <bluca@xxxxxxxxxx>

Alexander Mikhalitsyn (3):
  scm: add SO_PASSPIDFD and SCM_PIDFD
  net: core: add getsockopt SO_PEERPIDFD
  selftests: net: add SCM_PIDFD / SO_PEERPIDFD test

 arch/alpha/include/uapi/asm/socket.h          |   3 +
 arch/mips/include/uapi/asm/socket.h           |   3 +
 arch/parisc/include/uapi/asm/socket.h         |   3 +
 arch/sparc/include/uapi/asm/socket.h          |   3 +
 include/linux/net.h                           |   1 +
 include/linux/socket.h                        |   1 +
 include/net/scm.h                             |  39 +-
 include/uapi/asm-generic/socket.h             |   3 +
 net/core/sock.c                               |  44 ++
 net/mptcp/sockopt.c                           |   1 +
 net/unix/af_unix.c                            |  34 +-
 tools/include/uapi/asm-generic/socket.h       |   3 +
 tools/testing/selftests/net/.gitignore        |   1 +
 tools/testing/selftests/net/af_unix/Makefile  |   3 +-
 .../testing/selftests/net/af_unix/scm_pidfd.c | 430 ++++++++++++++++++
 15 files changed, 564 insertions(+), 8 deletions(-)
 create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c

-- 
2.34.1