Alban Crequy
9c444070ec
The source of the bind mount might not be accessible in a different user namespace because a component of the source path might not be traversed under the users and groups mapped inside the user namespace. This caused errors such as the following: # time="2020-06-22T13:48:26Z" level=error msg="container_linux.go:367: starting container process caused: process_linux.go:459: container init caused: rootfs_linux.go:58: mounting \"/tmp/busyboxtest/source-inaccessible/dir\" to rootfs at \"/tmp/inaccessible\" caused: stat /tmp/busyboxtest/source-inaccessible/dir: permission denied" To solve this problem, this patch performs the following: 1. in nsexec.c, it opens the source path in the host userns (so we have the right permissions to open it) but in the container mntns (so the kernel cross mntns mount check let us mount it later: https://github.com/torvalds/linux/blob/v5.8/fs/namespace.c#L2312). 2. in nsexec.c, it passes the file descriptors of the source to the child process with SCM_RIGHTS. 3. In runc-init in Golang, it finishes the mounts while inside the userns even without access to the some components of the source paths. Passing the fds with SCM_RIGHTS is necessary because once the child process is in the container mntns, it is already in the container userns so it cannot temporarily join the host mntns. This patch uses the existing mechanism with _LIBCONTAINER_* environment variables to pass the file descriptors from runc to runc init. This patch uses the existing mechanism with the Netlink-style bootstrap to pass information about the list of source mounts to nsexec.c. Rootless containers don't use this bind mount sources fdpassing mechanism because we can't setns() to the target mntns in a rootless container (we don't have the privileges when we are in the host userns). This patch takes care of using O_CLOEXEC on mount fds, and close them early. Fixes: #2484. Signed-off-by: Alban Crequy <alban@kinvolk.io> Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io> Co-authored-by: Rodrigo Campos <rodrigo@kinvolk.io>
nsenter
The nsenter package registers a special init constructor that is called before
the Go runtime has a chance to boot. This provides us the ability to setns on
existing namespaces and avoid the issues that the Go runtime has with multiple
threads. This constructor will be called if this package is registered,
imported, in your go application.
The nsenter package will import "C" and it uses cgo
package. In cgo, if the import of "C" is immediately preceded by a comment, that comment,
called the preamble, is used as a header when compiling the C parts of the package.
So every time we import package nsenter, the C code function nsexec() would be
called. And package nsenter is only imported in init.go, so every time the runc
init command is invoked, that C code is run.
Because nsexec() must be run before the Go runtime in order to use the
Linux kernel namespace, you must import this library into a package if
you plan to use libcontainer directly. Otherwise Go will not execute
the nsexec() constructor, which means that the re-exec will not cause
the namespaces to be joined. You can import it like this:
import _ "github.com/opencontainers/runc/libcontainer/nsenter"
nsexec() will first get the file descriptor number for the init pipe
from the environment variable _LIBCONTAINER_INITPIPE (which was opened
by the parent and kept open across the fork-exec of the nsexec() init
process). The init pipe is used to read bootstrap data (namespace paths,
clone flags, uid and gid mappings, and the console path) from the parent
process. nsexec() will then call setns(2) to join the namespaces
provided in the bootstrap data (if available), clone(2) a child process
with the provided clone flags, update the user and group ID mappings, do
some further miscellaneous setup steps, and then send the PID of the
child process to the parent of the nsexec() "caller". Finally,
the parent nsexec() will exit and the child nsexec() process will
return to allow the Go runtime take over.
NOTE: We do both setns(2) and clone(2) even if we don't have any
CLONE_NEW* clone flags because we must fork a new process in order to
enter the PID namespace.