From 3291d66b98445bd7f7d02eac7f2bca2ac2c56942 Mon Sep 17 00:00:00 2001 From: Aleksa Sarai Date: Sat, 21 Dec 2019 23:40:17 +1100 Subject: [PATCH] rootfs: do not permit /proc mounts to non-directories mount(2) will blindly follow symlinks, which is a problem because it allows a malicious container to trick runc into mounting /proc to an entirely different location (and thus within the attacker's control for a rename-exchange attack). This is just a hotfix (to "stop the bleeding"), and the more complete fix would be finish libpathrs and port runc to it (to avoid these types of attacks entirely, and defend against a variety of other /proc-related attacks). It can be bypased by someone having "/" be a volume controlled by another container. Fixes: CVE-2019-19921 Signed-off-by: Aleksa Sarai --- libcontainer/rootfs_linux.go | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/libcontainer/rootfs_linux.go b/libcontainer/rootfs_linux.go index 291021440..106c4c2b9 100644 --- a/libcontainer/rootfs_linux.go +++ b/libcontainer/rootfs_linux.go @@ -299,6 +299,18 @@ func mountToRootfs(m *configs.Mount, rootfs, mountLabel string, enableCgroupns b switch m.Device { case "proc", "sysfs": + // If the destination already exists and is not a directory, we bail + // out This is to avoid mounting through a symlink or similar -- which + // has been a "fun" attack scenario in the past. + // TODO: This won't be necessary once we switch to libpathrs and we can + // stop all of these symlink-exchange attacks. + if fi, err := os.Lstat(dest); err != nil { + if !os.IsNotExist(err) { + return err + } + } else if fi.Mode()&os.ModeDir == 0 { + return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device) + } if err := os.MkdirAll(dest, 0755); err != nil { return err }