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" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Kuniyuki Iwashima <[email protected]>
Cc: Lennart Poettering <[email protected]>
Cc: Luca Boccassi <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Stanislav Fomichev <[email protected]>
Tested-by: Luca Boccassi <[email protected]>
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