[PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

[Tycho Andersen]

This patch adds a way to insert FDs into the tracee's process (also
close/overwrite fds for the tracee). This functionality is necessary to
mock things like socketpair() or dup2() or similar, but since it depends on
external (vfs) patches, I've left it as a separate patch as before so the
core functionality can still be merged while we argue about this. Except
this time it doesn't add any ugliness to the API :)

v7: new in v7

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
CC: Christian Brauner <christian.brauner@xxxxxxxxxx>
CC: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
CC: Akihiro Suda <suda.akihiro@xxxxxxxxxxxxx>
---
 .../userspace-api/seccomp_filter.rst          |  16 +++
 include/uapi/linux/seccomp.h                  |   9 ++
 kernel/seccomp.c                              |  54 ++++++++
 tools/testing/selftests/seccomp/seccomp_bpf.c | 126 ++++++++++++++++++
 4 files changed, 205 insertions(+)

diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
index d2e61f1c0a0b..383a8dbae304 100644
--- a/Documentation/userspace-api/seccomp_filter.rst
+++ b/Documentation/userspace-api/seccomp_filter.rst
@@ -237,6 +237,13 @@ The interface for a seccomp notification fd consists of two structures:
         __s64 val;
     };
 
+    struct seccomp_notif_put_fd {
+        __u64 id;
+        __s32 fd;
+        __u32 fd_flags;
+        __s32 to_replace;
+    };
+
 Users can read via ``ioctl(SECCOMP_NOTIF_RECV)``  (or ``poll()``) on a seccomp
 notification fd to receive a ``struct seccomp_notif``, which contains five
 members: the input length of the structure, a unique-per-filter ``id``, the
@@ -256,6 +263,15 @@ mentioned above in this document: all arguments being read from the tracee's
 memory should be read into the tracer's memory before any policy decisions are
 made. This allows for an atomic decision on syscall arguments.
 
+Userspace can also insert (or overwrite) file descriptors of the tracee using
+``ioctl(SECCOMP_NOTIF_PUT_FD)``. The ``id`` member is the request/pid to insert
+the fd into. The ``fd`` is the fd in the listener's table to send or ``-1`` if
+an fd should be closed instead. The ``to_replace`` fd is the fd in the tracee's
+table that should be overwritten, or -1 if a new fd is installed. ``fd_flags``
+should be the flags that the fd in the tracee's table is opened with (e.g.
+``O_CLOEXEC`` or similar). The return value from this ioctl is the fd number
+that was installed.
+
 Sysctls
 =======
 
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index d4ccb32fe089..91d77f041fbb 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -77,6 +77,13 @@ struct seccomp_notif_resp {
 	__s64 val;
 };
 
+struct seccomp_notif_put_fd {
+	__u64 id;
+	__s32 fd;
+	__u32 fd_flags;
+	__s32 to_replace;
+};
+
 #define SECCOMP_IOC_MAGIC		0xF7
 
 /* Flags for seccomp notification fd ioctl. */
@@ -86,5 +93,7 @@ struct seccomp_notif_resp {
 					struct seccomp_notif_resp)
 #define SECCOMP_NOTIF_ID_VALID	_IOR(SECCOMP_IOC_MAGIC, 2,	\
 					__u64)
+#define SECCOMP_NOTIF_PUT_FD	_IOR(SECCOMP_IOC_MAGIC, 3,	\
+					struct seccomp_notif_put_fd)
 
 #endif /* _UAPI_LINUX_SECCOMP_H */
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 17685803a2af..07a05ad59731 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -41,6 +41,8 @@
 #include <linux/tracehook.h>
 #include <linux/uaccess.h>
 #include <linux/anon_inodes.h>
+#include <linux/fdtable.h>
+#include <net/cls_cgroup.h>
 
 enum notify_state {
 	SECCOMP_NOTIFY_INIT,
@@ -1684,6 +1686,56 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
 	return ret;
 }
 
+static long seccomp_notify_put_fd(struct seccomp_filter *filter,
+				  unsigned long arg)
+{
+	struct seccomp_notif_put_fd req;
+	void __user *buf = (void __user *)arg;
+	struct seccomp_knotif *knotif = NULL;
+	long ret;
+
+	if (copy_from_user(&req, buf, sizeof(req)))
+		return -EFAULT;
+
+	if (req.fd < 0 && req.to_replace < 0)
+		return -EINVAL;
+
+	ret = mutex_lock_interruptible(&filter->notify_lock);
+	if (ret < 0)
+		return ret;
+
+	ret = -ENOENT;
+	list_for_each_entry(knotif, &filter->notif->notifications, list) {
+		struct file *file = NULL;
+
+		if (knotif->id != req.id)
+			continue;
+
+		if (req.fd >= 0)
+			file = fget(req.fd);
+
+		if (req.to_replace >= 0) {
+			ret = replace_fd_task(knotif->task, req.to_replace,
+					      file, req.fd_flags);
+		} else {
+			unsigned long max_files;
+
+			max_files = task_rlimit(knotif->task, RLIMIT_NOFILE);
+			ret = __alloc_fd(knotif->task->files, 0, max_files,
+					 req.fd_flags);
+			if (ret < 0)
+				break;
+
+			__fd_install(knotif->task->files, ret, file);
+		}
+
+		break;
+	}
+
+	mutex_unlock(&filter->notify_lock);
+	return ret;
+}
+
 static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
 				 unsigned long arg)
 {
@@ -1696,6 +1748,8 @@ static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
 		return seccomp_notify_send(filter, arg);
 	case SECCOMP_NOTIF_ID_VALID:
 		return seccomp_notify_id_valid(filter, arg);
+	case SECCOMP_NOTIF_PUT_FD:
+		return seccomp_notify_put_fd(filter, arg);
 	default:
 		return -EINVAL;
 	}
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index c6ba3ed5392e..cd1322c02b92 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -43,6 +43,7 @@
 #include <sys/times.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <linux/kcmp.h>
 
 #include <unistd.h>
 #include <sys/syscall.h>
@@ -169,6 +170,9 @@ struct seccomp_metadata {
 					struct seccomp_notif_resp)
 #define SECCOMP_NOTIF_ID_VALID	_IOR(SECCOMP_IOC_MAGIC, 2,	\
 					__u64)
+#define SECCOMP_NOTIF_PUT_FD	_IOR(SECCOMP_IOC_MAGIC, 3,	\
+					struct seccomp_notif_put_fd)
+
 struct seccomp_notif {
 	__u16 len;
 	__u64 id;
@@ -183,6 +187,13 @@ struct seccomp_notif_resp {
 	__s32 error;
 	__s64 val;
 };
+
+struct seccomp_notif_put_fd {
+	__u64 id;
+	__s32 fd;
+	__u32 fd_flags;
+	__s32 to_replace;
+};
 #endif
 
 #ifndef seccomp
@@ -193,6 +204,14 @@ int seccomp(unsigned int op, unsigned int flags, void *args)
 }
 #endif
 
+#ifndef kcmp
+int kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1,
+	 unsigned long idx2)
+{
+	return syscall(__NR_kcmp, pid1, pid2, type, idx1, idx2);
+}
+#endif
+
 #ifndef PTRACE_SECCOMP_NEW_LISTENER
 #define PTRACE_SECCOMP_NEW_LISTENER 0x420e
 #endif
@@ -3243,6 +3262,113 @@ TEST(get_user_notification_ptrace)
 	close(listener);
 }
 
+TEST(user_notification_pass_fd)
+{
+	pid_t pid;
+	int status, listener, fd;
+	int sk_pair[2];
+	char c;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+	struct seccomp_notif_put_fd putfd = {};
+	long ret;
+
+	ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		char buf[16];
+
+		EXPECT_EQ(user_trap_syscall(__NR_getpid, 0), 0);
+
+		/* Signal we're ready and have installed the filter. */
+		EXPECT_EQ(write(sk_pair[1], "J", 1), 1);
+
+		EXPECT_EQ(read(sk_pair[1], &c, 1), 1);
+		EXPECT_EQ(c, 'H');
+		close(sk_pair[1]);
+
+		/* An fd from getpid(). Let the games begin. */
+		fd = syscall(__NR_getpid);
+		EXPECT_GT(fd, 0);
+		EXPECT_EQ(read(fd, buf, sizeof(buf)), 12);
+		close(fd);
+
+		exit(strcmp("hello world", buf));
+	}
+
+	EXPECT_EQ(read(sk_pair[0], &c, 1), 1);
+	EXPECT_EQ(c, 'J');
+
+	EXPECT_EQ(ptrace(PTRACE_ATTACH, pid), 0);
+	EXPECT_EQ(waitpid(pid, NULL, 0), pid);
+	listener = ptrace(PTRACE_SECCOMP_NEW_LISTENER, pid, 0);
+	EXPECT_GE(listener, 0);
+	EXPECT_EQ(ptrace(PTRACE_DETACH, pid, NULL, 0), 0);
+
+	/* Now signal we are done installing so it can do a getpid */
+	EXPECT_EQ(write(sk_pair[0], "H", 1), 1);
+	close(sk_pair[0]);
+
+	/* Make a new socket pair so we can send half across */
+	EXPECT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0);
+
+	ret = read_notif(listener, &req);
+	EXPECT_EQ(ret, sizeof(req));
+	EXPECT_EQ(errno, 0);
+
+	resp.len = sizeof(resp);
+	resp.id = req.id;
+
+	putfd.id = req.id;
+	putfd.fd_flags = 0;
+
+	/* First, let's just create a new fd with our stdout. */
+	putfd.fd = 0;
+	putfd.to_replace = -1;
+	fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+	EXPECT_GE(fd, 0);
+	EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, 0), 0);
+
+	/* Dup something else over the top of it. */
+	putfd.fd = sk_pair[1];
+	putfd.to_replace = fd;
+	fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+	EXPECT_GE(fd, 0);
+	EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 0);
+
+	/* Now, try to close it. */
+	putfd.fd = -1;
+	putfd.to_replace = fd;
+	fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+	EXPECT_GE(fd, 0);
+	EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 1);
+
+	/* Ok, we tried the three cases, now let's do what we really want. */
+	putfd.fd = sk_pair[1];
+	putfd.to_replace = -1;
+	fd = ioctl(listener, SECCOMP_NOTIF_PUT_FD, &putfd);
+	EXPECT_GE(fd, 0);
+	EXPECT_EQ(kcmp(req.pid, getpid(), KCMP_FILE, fd, sk_pair[1]), 0);
+
+	resp.val = fd;
+	resp.error = 0;
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_SEND, &resp), sizeof(resp));
+	close(sk_pair[1]);
+
+	EXPECT_EQ(write(sk_pair[0], "hello world\0", 12), 12);
+	close(sk_pair[0]);
+
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status));
+	close(listener);
+}
+
 /*
  * Check that a pid in a child namespace still shows up as valid in ours.
  */
-- 
2.17.1