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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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