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From:   Sargun Dhillon <sargun@sargun.me>
To:     linux-kernel@vger.kernel.org,
        containers@lists.linux-foundation.org, linux-api@vger.kernel.org
Cc:     Sargun Dhillon <sargun@sargun.me>, christian.brauner@ubuntu.com,
        tycho@tycho.ws, keescook@chromium.org, cyphar@cyphar.com,
        Jeffrey Vander Stoep <jeffv@google.com>, jannh@google.com,
        rsesek@google.com, palmer@google.com,
        Matt Denton <mpdenton@google.com>,
        Kees Cook <keescook@google.com>
Subject: [PATCH 4/5] seccomp: Add SECCOMP_ADDFD_FLAG_MOVE flag to add fd ioctl
Date:   Sun, 24 May 2020 16:39:41 -0700
Message-Id: <20200524233942.8702-5-sargun@sargun.me>
In-Reply-To: <20200524233942.8702-1-sargun@sargun.me>
References: <20200524233942.8702-1-sargun@sargun.me>
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Sender: linux-kernel-owner@vger.kernel.org
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Certain files, when moved to another process have metadata changed, such
as netprioidx, and classid. This is the default behaviour in sending
sockets with SCM_RIGHTS over unix sockets. Depending on the usecase,
this may or may not be desirable with the addfd ioctl. This allows
the user to opt-in.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Tycho Andersen <tycho@tycho.ws>
Cc: Matt Denton <mpdenton@google.com>
Cc: Kees Cook <keescook@google.com>,
Cc: Jann Horn <jannh@google.com>,
Cc: Robert Sesek <rsesek@google.com>,
Cc: Chris Palmer <palmer@google.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
---
 include/uapi/linux/seccomp.h |  8 ++++++++
 kernel/seccomp.c             | 31 +++++++++++++++++++++++++++----
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index 7d450a9e4c29..ccd1c960372a 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -115,6 +115,14 @@ struct seccomp_notif_resp {
 
 /* valid flags for seccomp_notif_addfd */
 #define SECCOMP_ADDFD_FLAG_SETFD	(1UL << 0) /* Specify remote fd */
+/*
+ * Certain file descriptors are behave differently depending on the process
+ * they are created in. Specifcally, sockets, and their interactions with the
+ * net_cls and net_prio cgroup v1 controllers. This "moves" the file descriptor
+ * so that it takes on the cgroup controller's configuration in the process
+ * that the file descriptor is being added to.
+ */
+#define SECCOMP_ADDFD_FLAG_MOVE		(1UL << 1)
 
 /**
  * struct seccomp_notif_addfd
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 88940eeabaee..2e649f3cb10e 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -41,6 +41,9 @@
 #include <linux/tracehook.h>
 #include <linux/uaccess.h>
 #include <linux/anon_inodes.h>
+#include <net/netprio_cgroup.h>
+#include <net/sock.h>
+#include <net/cls_cgroup.h>
 
 enum notify_state {
 	SECCOMP_NOTIFY_INIT,
@@ -108,6 +111,7 @@ struct seccomp_kaddfd {
 	struct file *file;
 	int fd;
 	unsigned int flags;
+	bool move;
 
 	/* To only be set on reply */
 	int ret;
@@ -769,7 +773,8 @@ static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
 
 static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
 {
-	int ret;
+	struct socket *sock;
+	int err, ret;
 
 	/*
 	 * Remove the notification, and reset the list pointers, indicating
@@ -785,12 +790,29 @@ static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
 		ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
 		if (ret >= 0)
 			fput(addfd->file);
+		else
+			goto out;
 	} else {
 		ret = get_unused_fd_flags(addfd->flags);
 		if (ret >= 0)
 			fd_install(ret, addfd->file);
+		else
+			goto out;
 	}
 
+	if (addfd->move) {
+		sock = sock_from_file(addfd->file, &err);
+		if (sock) {
+			sock_update_netprioidx(&sock->sk->sk_cgrp_data);
+			sock_update_classid(&sock->sk->sk_cgrp_data);
+		}
+	}
+	/*
+	 * An extra reference is taken on the ioctl side, so upon success, we
+	 * must consume all references (and on failure, none).
+	 */
+	fput(addfd->file);
+
 out:
 	addfd->ret = ret;
 	complete(&addfd->completion);
@@ -1279,16 +1301,17 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter,
 	if (addfd.fd_flags & (~O_CLOEXEC))
 		return -EINVAL;
 
-	if (addfd.flags & ~(SECCOMP_ADDFD_FLAG_SETFD))
+	if (addfd.flags & ~(SECCOMP_ADDFD_FLAG_SETFD|SECCOMP_ADDFD_FLAG_MOVE))
 		return -EINVAL;
 
 	if (addfd.remote_fd && !(addfd.flags & SECCOMP_ADDFD_FLAG_SETFD))
 		return -EINVAL;
 
-	kaddfd.file = fget(addfd.fd);
+	kaddfd.file = fget_many(addfd.fd, 2);
 	if (!kaddfd.file)
 		return -EBADF;
 
+	kaddfd.move = (addfd.flags & SECCOMP_ADDFD_FLAG_MOVE);
 	kaddfd.flags = addfd.fd_flags;
 	kaddfd.fd = (addfd.flags & SECCOMP_ADDFD_FLAG_SETFD) ?
 		    addfd.remote_fd : -1;
@@ -1339,7 +1362,7 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter,
 	mutex_unlock(&filter->notify_lock);
 out:
 	if (ret < 0)
-		fput(kaddfd.file);
+		fput_many(kaddfd.file, 2);
 
 	return ret;
 }
-- 
2.25.1