Christian Brauner
bb7d8b1f41
My first attempt to simplify this and make it less costly focussed on the way constructors are called. I was under the impression that the ELF specification mandated that arg, argv, and actually even envp need to be passed to functions located in the .init_arry section (aka "constructors"). Actually, the specifications is (cf. [2]): SHT_INIT_ARRAY This section contains an array of pointers to initialization functions, as described in ``Initialization and Termination Functions'' in Chapter 5. Each pointer in the array is taken as a parameterless procedure with a void return. which means that this becomes a libc specific decision. Glibc passes down those args, musl doesn't. So this approach can't work. However, we can at least remove the environment parsing part based on POSIX since [1] mandates that there should be an environ variable defined in unistd.h which provides access to the environment. See also the relevant Open Group specification [1]. [1]: http://pubs.opengroup.org/onlinepubs/9699919799/ [2]: http://www.sco.com/developers/gabi/latest/ch4.sheader.html#init_array Fixes: CVE-2019-5736 Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
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.