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From 28a697cce3e4f905dca700eda81d681a30eef9cd Mon Sep 17 00:00:00 2001
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From: Giuseppe Scrivano <gscrivan@redhat.com>
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Date: Fri, 11 Jan 2019 21:53:45 +0100
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Subject: [PATCH] rootfs: umount all procfs and sysfs with --no-pivot
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When creating a new user namespace, the kernel doesn't allow to mount
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a new procfs or sysfs file system if there is not already one instance
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fully visible in the current mount namespace.
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When using --no-pivot we were effectively inhibiting this protection
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from the kernel, as /proc and /sys from the host are still present in
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the container mount namespace.
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A container without full access to /proc could then create a new user
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namespace, and from there able to mount a fully visible /proc, bypassing
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the limitations in the container.
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A simple reproducer for this issue is:
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unshare -mrfp sh -c "mount -t proc none /proc && echo c > /proc/sysrq-trigger"
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Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
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---
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libcontainer/rootfs_linux.go | 35 +++++++++++++++++++++++++++++++++++
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1 file changed, 35 insertions(+)
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diff --git a/libcontainer/rootfs_linux.go b/libcontainer/rootfs_linux.go
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index e7c2f8ada..6bd6da74a 100644
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--- a/libcontainer/rootfs_linux.go
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+++ b/libcontainer/rootfs_linux.go
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@@ -748,6 +748,41 @@ func pivotRoot(rootfs string) error {
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}
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func msMoveRoot(rootfs string) error {
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+ mountinfos, err := mount.GetMounts()
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+ if err != nil {
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+ return err
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+ }
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+
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+ absRootfs, err := filepath.Abs(rootfs)
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+ if err != nil {
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+ return err
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+ }
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+
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+ for _, info := range mountinfos {
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+ p, err := filepath.Abs(info.Mountpoint)
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+ if err != nil {
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+ return err
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+ }
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+ // Umount every syfs and proc file systems, except those under the container rootfs
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+ if (info.Fstype != "proc" && info.Fstype != "sysfs") || filepath.HasPrefix(p, absRootfs) {
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+ continue
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+ }
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+ // Be sure umount events are not propagated to the host.
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+ if err := unix.Mount("", p, "", unix.MS_SLAVE|unix.MS_REC, ""); err != nil {
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+ return err
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+ }
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+ if err := unix.Unmount(p, unix.MNT_DETACH); err != nil {
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+ if err != unix.EINVAL && err != unix.EPERM {
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+ return err
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+ } else {
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+ // If we have not privileges for umounting (e.g. rootless), then
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+ // cover the path.
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+ if err := unix.Mount("tmpfs", p, "tmpfs", 0, ""); err != nil {
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+ return err
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+ }
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+ }
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+ }
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+ }
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if err := unix.Mount(rootfs, "/", "", unix.MS_MOVE, ""); err != nil {
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return err
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}
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