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Message-Id: <20230522132439.634031-1-aleksandr.mikhalitsyn@canonical.com> Date: Mon, 22 May 2023 15:24:36 +0200 From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com> To: davem@...emloft.net Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Leon Romanovsky <leon@...nel.org>, David Ahern <dsahern@...nel.org>, Arnd Bergmann <arnd@...db.de>, Kees Cook <keescook@...omium.org>, Christian Brauner <brauner@...nel.org>, Kuniyuki Iwashima <kuniyu@...zon.com>, Lennart Poettering <mzxreary@...inter.de>, Luca Boccassi <bluca@...ian.org>, Daniel Borkmann <daniel@...earbox.net>, Stanislav Fomichev <sdf@...gle.com> Subject: [PATCH net-next v6 0/3] Add SCM_PIDFD and SO_PEERPIDFD 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@...emloft.net> Cc: Eric Dumazet <edumazet@...gle.com> Cc: Jakub Kicinski <kuba@...nel.org> Cc: Paolo Abeni <pabeni@...hat.com> Cc: Leon Romanovsky <leon@...nel.org> Cc: David Ahern <dsahern@...nel.org> Cc: Arnd Bergmann <arnd@...db.de> Cc: Kees Cook <keescook@...omium.org> Cc: Christian Brauner <brauner@...nel.org> Cc: Kuniyuki Iwashima <kuniyu@...zon.com> Cc: Lennart Poettering <mzxreary@...inter.de> Cc: Luca Boccassi <bluca@...ian.org> Cc: Daniel Borkmann <daniel@...earbox.net> Cc: Stanislav Fomichev <sdf@...gle.com> Tested-by: Luca Boccassi <bluca@...ian.org> 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 | 43 +- include/uapi/asm-generic/socket.h | 3 + net/core/sock.c | 48 ++ net/mptcp/sockopt.c | 3 + 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, 574 insertions(+), 8 deletions(-) create mode 100644 tools/testing/selftests/net/af_unix/scm_pidfd.c -- 2.34.1
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