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Message-ID: <20250510015652.9931-9-kuniyu@amazon.com> Date: Fri, 9 May 2025 18:56:31 -0700 From: Kuniyuki Iwashima <kuniyu@...zon.com> To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Willem de Bruijn <willemb@...gle.com> CC: Simon Horman <horms@...nel.org>, Christian Brauner <brauner@...nel.org>, Kuniyuki Iwashima <kuniyu@...zon.com>, Kuniyuki Iwashima <kuni1840@...il.com>, <netdev@...r.kernel.org> Subject: [PATCH v2 net-next 8/9] 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 <kuniyu@...zon.com> --- arch/alpha/include/uapi/asm/socket.h | 2 ++ arch/mips/include/uapi/asm/socket.h | 2 ++ arch/parisc/include/uapi/asm/socket.h | 2 ++ arch/sparc/include/uapi/asm/socket.h | 2 ++ include/net/sock.h | 4 +++- include/uapi/asm-generic/socket.h | 2 ++ net/core/sock.c | 13 +++++++++++++ net/unix/af_unix.c | 22 ++++++++++++++++++++-- tools/include/uapi/asm-generic/socket.h | 2 ++ 9 files changed, 48 insertions(+), 3 deletions(-) diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h index 3df5f2dd4c0f..8f1f18adcdb5 100644 --- a/arch/alpha/include/uapi/asm/socket.h +++ b/arch/alpha/include/uapi/asm/socket.h @@ -150,6 +150,8 @@ #define SO_RCVPRIORITY 82 +#define SO_PASSRIGHTS 83 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h index 22fa8f19924a..31ac655b7837 100644 --- a/arch/mips/include/uapi/asm/socket.h +++ b/arch/mips/include/uapi/asm/socket.h @@ -161,6 +161,8 @@ #define SO_RCVPRIORITY 82 +#define SO_PASSRIGHTS 83 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h index 96831c988606..1f2d5b7a7f5d 100644 --- a/arch/parisc/include/uapi/asm/socket.h +++ b/arch/parisc/include/uapi/asm/socket.h @@ -142,6 +142,8 @@ #define SCM_DEVMEM_DMABUF SO_DEVMEM_DMABUF #define SO_DEVMEM_DONTNEED 0x4050 +#define SO_PASSRIGHTS 0x4051 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h index 5b464a568664..adcba7329386 100644 --- a/arch/sparc/include/uapi/asm/socket.h +++ b/arch/sparc/include/uapi/asm/socket.h @@ -143,6 +143,8 @@ #define SO_RCVPRIORITY 0x005b +#define SO_PASSRIGHTS 0x005c + #if !defined(__KERNEL__) diff --git a/include/net/sock.h b/include/net/sock.h index 036ed7d394ba..26c7d85df7d3 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 * @ns_tracker: tracker for netns reference * @sk_user_frags: xarray of pages the user is holding a reference on. * @sk_owner: reference to the real owner of the socket that calls @@ -533,7 +534,8 @@ 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; u8 sk_txtime_deadline_mode : 1, diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h index aa5016ff3d91..f333a0ac4ee4 100644 --- a/include/uapi/asm-generic/socket.h +++ b/include/uapi/asm-generic/socket.h @@ -145,6 +145,8 @@ #define SO_RCVPRIORITY 82 +#define SO_PASSRIGHTS 83 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__)) diff --git a/net/core/sock.c b/net/core/sock.c index 9540cbe3d83e..c9f81019cb9d 100644 --- a/net/core/sock.c +++ b/net/core/sock.c @@ -1238,6 +1238,12 @@ int sk_setsockopt(struct sock *sk, int level, int optname, sk->sk_scm_pidfd = valbool; return 0; + case SO_PASSRIGHTS: + if (!sk_is_unix(sk)) + return -EOPNOTSUPP; + + sk->sk_scm_rights = valbool; + return 0; case SO_TYPE: case SO_PROTOCOL: case SO_DOMAIN: @@ -1877,6 +1883,13 @@ int sk_getsockopt(struct sock *sk, int level, int optname, v.val = 0; break; + case SO_PASSRIGHTS: + if (sk_is_unix(sk)) + v.val = sk->sk_scm_rights; + else + v.val = 0; + break; + case SO_PEERCRED: { struct ucred peercred; diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index ba52fc36f9be..941098b090ef 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -1015,6 +1015,7 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern, sock_init_data(sock, sk); + sk->sk_scm_rights = 1; sk->sk_hash = unix_unbound_hash(sk); sk->sk_allocation = GFP_KERNEL_ACCOUNT; sk->sk_write_space = unix_write_space; @@ -2073,6 +2074,11 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg, goto out_unlock; } + if (UNIXCB(skb).fp && !other->sk_scm_rights) { + err = -EPERM; + goto out_unlock; + } + if (sk->sk_type != SOCK_SEQPACKET) { err = security_unix_may_send(sk->sk_socket, other->sk_socket); if (err) @@ -2174,9 +2180,13 @@ static int queue_oob(struct sock *sk, struct msghdr *msg, struct sock *other, if (sock_flag(other, SOCK_DEAD) || (other->sk_shutdown & RCV_SHUTDOWN)) { - unix_state_unlock(other); err = -EPIPE; - goto out; + goto out_unlock; + } + + if (UNIXCB(skb).fp && !other->sk_scm_rights) { + err = -EPERM; + goto out_unlock; } unix_maybe_add_creds(skb, sk, other); @@ -2192,6 +2202,8 @@ static int queue_oob(struct sock *sk, struct msghdr *msg, struct sock *other, other->sk_data_ready(other); return 0; +out_unlock: + unix_state_unlock(other); out: consume_skb(skb); return err; @@ -2295,6 +2307,12 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg, (other->sk_shutdown & RCV_SHUTDOWN)) goto out_pipe_unlock; + if (UNIXCB(skb).fp && !other->sk_scm_rights) { + unix_state_unlock(other); + err = -EPERM; + goto out_free; + } + unix_maybe_add_creds(skb, sk, other); scm_stat_add(other, skb); skb_queue_tail(&other->sk_receive_queue, skb); diff --git a/tools/include/uapi/asm-generic/socket.h b/tools/include/uapi/asm-generic/socket.h index aa5016ff3d91..f333a0ac4ee4 100644 --- a/tools/include/uapi/asm-generic/socket.h +++ b/tools/include/uapi/asm-generic/socket.h @@ -145,6 +145,8 @@ #define SO_RCVPRIORITY 82 +#define SO_PASSRIGHTS 83 + #if !defined(__KERNEL__) #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__)) -- 2.49.0
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