From 77cbe1a6d8730a07f99f9263c2d5f2304cf5e830 Mon Sep 17 00:00:00 2001 From: Kuniyuki Iwashima Date: Mon, 19 May 2025 13:57:59 -0700 Subject: af_unix: Introduce SO_PASSRIGHTS. As long as recvmsg() or recvmmsg() is used with cmsg, it is not possible to avoid receiving file descriptors via SCM_RIGHTS. This behaviour has occasionally been flagged as problematic, as it can be (ab)used to trigger DoS during close(), for example, by passing a FUSE-controlled fd or a hung NFS fd. For instance, as noted on the uAPI Group page [0], an untrusted peer could send a file descriptor pointing to a hung NFS mount and then close it. Once the receiver calls recvmsg() with msg_control, the descriptor is automatically installed, and then the responsibility for the final close() now falls on the receiver, which may result in blocking the process for a long time. Regarding this, systemd calls cmsg_close_all() [1] after each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS. However, this cannot work around the issue at all, because the final fput() may still occur on the receiver's side once sendmsg() with SCM_RIGHTS succeeds. Also, even filtering by LSM at recvmsg() does not work for the same reason. Thus, we need a better way to refuse SCM_RIGHTS at sendmsg(). Let's introduce SO_PASSRIGHTS to disable SCM_RIGHTS. Note that this option is enabled by default for backward compatibility. Link: https://uapi-group.org/kernel-features/#disabling-reception-of-scm_rights-for-af_unix-sockets #[0] Link: https://github.com/systemd/systemd/blob/v257.5/src/basic/fd-util.c#L612-L628 #[1] Signed-off-by: Kuniyuki Iwashima Reviewed-by: Willem de Bruijn Signed-off-by: David S. Miller --- include/net/sock.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include/net') diff --git a/include/net/sock.h b/include/net/sock.h index d90a71f66ab8..92e7c1aae3cc 100644 --- a/include/net/sock.h +++ b/include/net/sock.h @@ -341,6 +341,7 @@ struct sk_filter; * @sk_scm_credentials: flagged by SO_PASSCRED to recv SCM_CREDENTIALS * @sk_scm_security: flagged by SO_PASSSEC to recv SCM_SECURITY * @sk_scm_pidfd: flagged by SO_PASSPIDFD to recv SCM_PIDFD + * @sk_scm_rights: flagged by SO_PASSRIGHTS to recv SCM_RIGHTS * @sk_scm_unused: unused flags for scm_recv() * @ns_tracker: tracker for netns reference * @sk_user_frags: xarray of pages the user is holding a reference on. @@ -535,7 +536,8 @@ struct sock { u8 sk_scm_credentials : 1, sk_scm_security : 1, sk_scm_pidfd : 1, - sk_scm_unused : 5; + sk_scm_rights : 1, + sk_scm_unused : 4; }; }; u8 sk_clockid; -- cgit v1.2.3