userns: Don't allow creation if the user is chrooted (3151527e) · Commits · 戴 / test

Guarantee that the policy of which files may be access that is established by setting the root directory will not be violated by user namespaces by verifying that the root directory points to the root of the mount namespace at the time of user namespace creation. Changing the root is a privileged operation, and as a matter of policy it serves to limit unprivileged processes to files below the current root directory. For reasons of simplicity and comprehensibility the privilege to change the root directory is gated solely on the CAP_SYS_CHROOT capability in the user namespace. Therefore when creating a user namespace we must ensure that the policy of which files may be access can not be violated by changing the root directory. Anyone who runs a processes in a chroot and would like to use user namespace can setup the same view of filesystems with a mount namespace instead. With this result that this is not a practical limitation for using user namespaces. Cc: stable@vger.kernel.org Acked-by:

Serge Hallyn <serge.hallyn@canonical.com> Reported-by:

Andy Lutomirski <luto@amacapital.net> Signed-off-by:

"Eric W. Biederman" <ebiederm@xmission.com>

fs/namespace.c

+24 −0

Original line number	Diff line number	Diff line
		@@ -2732,6 +2732,30 @@ bool our_mnt(struct vfsmount *mnt)
		return check_mnt(real_mount(mnt));
		}

		bool current_chrooted(void)
		{
		/* Does the current process have a non-standard root */
		struct path ns_root;
		struct path fs_root;
		bool chrooted;

		/* Find the namespace root */
		ns_root.mnt = &current->nsproxy->mnt_ns->root->mnt;
		ns_root.dentry = ns_root.mnt->mnt_root;
		path_get(&ns_root);
		while (d_mountpoint(ns_root.dentry) && follow_down_one(&ns_root))
		;

		get_fs_root(current->fs, &fs_root);

		chrooted = !path_equal(&fs_root, &ns_root);

		path_put(&fs_root);
		path_put(&ns_root);

		return chrooted;
		}

		static void mntns_get(struct task_struct task)
		{
		struct mnt_namespace *ns = NULL;

include/linux/fs_struct.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -50,4 +50,6 @@ static inline void get_fs_root_and_pwd(struct fs_struct fs, struct path root,
		spin_unlock(&fs->lock);
		}

		extern bool current_chrooted(void);

		#endif /* _LINUX_FS_STRUCT_H */

kernel/user_namespace.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -61,6 +61,15 @@ int create_user_ns(struct cred *new)
		kgid_t group = new->egid;
		int ret;

		/*
		* Verify that we can not violate the policy of which files
		* may be accessed that is specified by the root directory,
		* by verifing that the root directory is at the root of the
		* mount namespace which allows all files to be accessed.
		*/
		if (current_chrooted())
		return -EPERM;

		/* The creator needs a mapping in the parent user namespace
		* or else we won't be able to reasonably tell userspace who
		* created a user_namespace.

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