bus: when connecting to a container's kdbus instance, enter namespace first

Previously we'd open the connection in the originating namespace, which
meant most peers of the bus would not be able to make sense of the
PID/UID/... identity of us since we didn't exist in the namespace they
run in. However they require this identity for privilege decisions,
hence disallowing access to anything from the host.

Instead, when connecting to a container, create a temporary subprocess,
make it join the container's namespace and then connect from there to
the kdbus instance. This is similar to how we do it for socket
conections already.

THis also unifies the namespacing code used by machinectl and the bus
APIs.

1 files changed, 12 insertions, 29 deletions

diff --git a/src/machine/machinectl.c b/src/machine/machinectl.c
index 7bb7086056..f5485b3d42 100644
--- a/src/machine/machinectl.c
+++ b/src/machine/machinectl.c
@@ -398,8 +398,8 @@ static int terminate_machine(sd_bus *bus, char **args, unsigned n) {
 }
 
 static int openpt_in_namespace(pid_t pid, int flags) {
+        _cleanup_close_pipe_ int pair[2] = { -1, -1 };
         _cleanup_close_ int nsfd = -1, rootfd = -1;
-        _cleanup_close_pipe_ int sock[2] = { -1, -1 };
         union {
                 struct cmsghdr cmsghdr;
                 uint8_t buf[CMSG_SPACE(sizeof(int))];
@@ -410,23 +410,14 @@ static int openpt_in_namespace(pid_t pid, int flags) {
         };
         struct cmsghdr *cmsg;
         int master = -1, r;
-        char *ns, *root;
         pid_t child;
         siginfo_t si;
 
-        ns = procfs_file_alloca(pid, "ns/mnt");
-
-        nsfd = open(ns, O_RDONLY|O_NOCTTY|O_CLOEXEC);
-        if (nsfd < 0)
-                return -errno;
-
-        root = procfs_file_alloca(pid, "root");
-
-        rootfd = open(root, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY);
-        if (rootfd < 0)
-                return -errno;
+        r = namespace_open(pid, &nsfd, &rootfd);
+        if (r < 0)
+                return r;
 
-        if (socketpair(AF_UNIX, SOCK_DGRAM, 0, sock) < 0)
+        if (socketpair(AF_UNIX, SOCK_DGRAM, 0, pair) < 0)
                 return -errno;
 
         child = fork();
@@ -434,19 +425,13 @@ static int openpt_in_namespace(pid_t pid, int flags) {
                 return -errno;
 
         if (child == 0) {
-                close_nointr_nofail(sock[0]);
-                sock[0] = -1;
+                close_nointr_nofail(pair[0]);
+                pair[0] = -1;
 
-                r = setns(nsfd, CLONE_NEWNS);
+                r = namespace_enter(nsfd, rootfd);
                 if (r < 0)
                         _exit(EXIT_FAILURE);
 
-                if (fchdir(rootfd) < 0)
-                        _exit(EXIT_FAILURE);
-
-                if (chroot(".") < 0)
-                        _exit(EXIT_FAILURE);
-
                 master = posix_openpt(flags);
                 if (master < 0)
                         _exit(EXIT_FAILURE);
@@ -459,18 +444,16 @@ static int openpt_in_namespace(pid_t pid, int flags) {
 
                 mh.msg_controllen = cmsg->cmsg_len;
 
-                r = sendmsg(sock[1], &mh, MSG_NOSIGNAL);
-                close_nointr_nofail(master);
-                if (r < 0)
+                if (sendmsg(pair[1], &mh, MSG_NOSIGNAL) < 0)
                         _exit(EXIT_FAILURE);
 
                 _exit(EXIT_SUCCESS);
         }
 
-        close_nointr_nofail(sock[1]);
-        sock[1] = -1;
+        close_nointr_nofail(pair[1]);
+        pair[1] = -1;
 
-        if (recvmsg(sock[0], &mh, MSG_NOSIGNAL|MSG_CMSG_CLOEXEC) < 0)
+        if (recvmsg(pair[0], &mh, MSG_NOSIGNAL|MSG_CMSG_CLOEXEC) < 0)
                 return -errno;
 
         for (cmsg = CMSG_FIRSTHDR(&mh); cmsg; cmsg = CMSG_NXTHDR(&mh, cmsg))