[PATCH net-next v3 3/5] net: af_unix: useful handling of LSM denials on SCM_RIGHTS
From: Jori Koolstra <jkoolstra@xs4all.nl>
Date: 2026-06-29 19:42:32
Also in:
linux-fsdevel, lkml
Subsystem:
generic include/asm header files, networking [general], networking [sockets], networking [unix sockets], the rest · Maintainers:
Arnd Bergmann, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Kuniyuki Iwashima, Willem de Bruijn, Linus Torvalds
Right now if some LSM such as Smack denies an AF_UNIX socket peer to receive an SCM_RIGHTS fd, the SCM_RIGHTS fd array will be cut short at that point, and MSG_CTRUNC is set on return of recvmsg(). This is highly problematic behaviour, because it leaves the receiver wondering what happened. As per man page MSG_CTRUNC is supposed to indicate that the control buffer was sized too short, but suddenly a permission error might result in the exact same flag being set. Moreover, the receiver has no chance to determine how many fds got originally sent and how many were suppressed.[1] Add a SO_RIGHTS_NOTRUNC option to UNIX sockets to enable more useful handling of LSM denials when receiving SCM_RIGHTS messages: instead of truncating the message at the first blocked fd, keep every fd slot and store the LSM errno in the blocked slot. [1]: https://github.com/uapi-group/kernel-features#useful-handling-of-lsm-denials-on-scm_rights Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl> --- include/net/af_unix.h | 1 + include/net/scm.h | 15 +++++++++++---- include/uapi/asm-generic/socket.h | 3 +++ net/compat.c | 4 ++-- net/core/scm.c | 16 +++++++++++----- net/unix/af_unix.c | 9 +++++++++ 6 files changed, 37 insertions(+), 11 deletions(-)
diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 34f53dde65ce..bb1b3dee02e8 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h@@ -49,6 +49,7 @@ struct unix_sock { struct scm_stat scm_stat; int inq_len; bool recvmsg_inq; + bool scm_rights_notrunc; #if IS_ENABLED(CONFIG_AF_UNIX_OOB) struct sk_buff *oob_skb; #endif
diff --git a/include/net/scm.h b/include/net/scm.h
index c52519669349..761cda0803fb 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h@@ -50,8 +50,8 @@ struct scm_cookie { #endif }; -void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm); -void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm); +void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm, bool notrunc); +void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm, bool notrunc); int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm); void __scm_destroy(struct scm_cookie *scm); struct scm_fp_list *scm_fp_dup(struct scm_fp_list *fpl);
@@ -108,11 +108,18 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm, int flags); static inline int scm_recv_one_fd(struct file *f, int __user *ufd, - unsigned int flags) + unsigned int flags, bool notrunc) { + bool filtered; + int error; + if (!ufd) return -EFAULT; - return receive_fd(f, ufd, flags); + + error = receive_fd_filtered(f, ufd, flags, &filtered); + if (filtered && notrunc) + return put_user(error, ufd); + return error; } #endif /* __LINUX_NET_SCM_H */
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 53b5a8c002b1..c5fb2ee96830 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h@@ -150,6 +150,9 @@ #define SO_INQ 84 #define SCM_INQ SO_INQ +#define SO_RIGHTS_NOTRUNC 85 +#define SCM_RIGHTS_NOTRUNC SO_RIGHTS_NOTRUNC + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
diff --git a/net/compat.c b/net/compat.c
index d68cf9c3aad5..6bdf4a2c9077 100644
--- a/net/compat.c
+++ b/net/compat.c@@ -286,7 +286,7 @@ static int scm_max_fds_compat(struct msghdr *msg) return (msg->msg_controllen - sizeof(struct compat_cmsghdr)) / sizeof(int); } -void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm) +void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm, bool notrunc) { struct compat_cmsghdr __user *cm = (struct compat_cmsghdr __user *)msg->msg_control_user;
@@ -296,7 +296,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm) int err = 0, i; for (i = 0; i < fdmax; i++) { - err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags); + err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags, notrunc); if (err < 0) break; }
diff --git a/net/core/scm.c b/net/core/scm.c
index a73b1eb30fd2..55bab203281a 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c@@ -351,7 +351,7 @@ static int scm_max_fds(struct msghdr *msg) return (msg->msg_controllen - sizeof(struct cmsghdr)) / sizeof(int); } -void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm) +void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm, bool notrunc) { struct cmsghdr __user *cm = (__force struct cmsghdr __user *)msg->msg_control_user;
@@ -365,12 +365,12 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm) return; if (msg->msg_flags & MSG_CMSG_COMPAT) { - scm_detach_fds_compat(msg, scm); + scm_detach_fds_compat(msg, scm, notrunc); return; } for (i = 0; i < fdmax; i++) { - err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags); + err = scm_recv_one_fd(scm->fp->fp[i], cmsg_data + i, o_flags, notrunc); if (err < 0) break; }
@@ -542,8 +542,14 @@ void scm_recv_unix(struct socket *sock, struct msghdr *msg, if (!__scm_recv_common(sock->sk, msg, scm, flags)) return; - if (scm->fp) - scm_detach_fds(msg, scm); + if (scm->fp) { + struct unix_sock *u; + bool notrunc; + + u = unix_sk(sock->sk); + notrunc = READ_ONCE(u->scm_rights_notrunc); + scm_detach_fds(msg, scm, notrunc); + } if (sock->sk->sk_scm_pidfd) scm_pidfd_recv(msg, scm);
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f7a9d55eee8a..83274ce18e06 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c@@ -921,6 +921,7 @@ static bool unix_custom_sockopt(int optname) { switch (optname) { case SO_INQ: + case SO_RIGHTS_NOTRUNC: return true; default: return false;
@@ -956,6 +957,14 @@ static int unix_setsockopt(struct socket *sock, int level, int optname, WRITE_ONCE(u->recvmsg_inq, val); break; + + case SO_RIGHTS_NOTRUNC: + if (val > 1 || val < 0) + return -EINVAL; + + WRITE_ONCE(u->scm_rights_notrunc, val); + break; + default: return -ENOPROTOOPT; }
--
2.54.0