Re: [PATCH 4/7] fs: Treat foreign mounts as nosuid

Nikolay Borisov
2015-07-17T06:46:20Z

On 07/15/2015 10:46 PM, Seth Forshee wrote:
> From: Andy Lutomirski <luto@amacapital.net>
>
> If a process gets access to a mount from a different namespace user
> namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem.
> Technically, trusting mounts created by the same or ancestor user
> namespaces ought to be safe, but it's simpler to distrust all
> foreign mounts.
>
> This will make it safer to allow more complex filesystems to be
> mounted in non-root user namespaces.
>
> This does not remove the need for MNT_LOCK_NOSUID. The setuid,
> setgid, and file capability bits can no longer be abused if code in
> a user namespace were to clear nosuid on an untrusted filesystem,
> but this patch, by itself, is insufficient to protect the system
> from abuse of files that, when execed, would increase MAC privilege.
>
> As a more concrete explanation, any task that can manipulate a
> vfsmount associated with a given user namespace already has
> capabilities in that namespace and all of its descendents. If they
> can cause a malicious setuid, setgid, or file-caps executable to
> appear in that mount, then that executable will only allow them to
> elevate privileges in exactly the set of namespaces in which they
> are already privileges.
>
> On the other hand, if they can cause a malicious executable to
> appear with a dangerous MAC label, running it could change the
> caller's security context in a way that should not have been
> possible, even inside the namespace in which the task is confined.
>
> As a hardening measure, this would have made CVE-2014-5207 much
> more difficult to exploit.
>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> [ saf: Forward ported to 4.2 ]
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> ---
> fs/exec.c | 2 +-
> fs/namespace.c | 13 +++++++++++++
> include/linux/mount.h | 1 +
> security/commoncap.c | 2 +-
> security/selinux/hooks.c | 2 +-
> 5 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index b06623a9347f..ea7311d72cc3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1295,7 +1295,7 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
> bprm->cred->euid = current_euid();
> bprm->cred->egid = current_egid();
>
> - if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> + if (!mnt_may_suid(bprm->file->f_path.mnt))
> return;
>
> if (task_no_new_privs(current))
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 423001de32a2..2bfd7ca92247 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3252,6 +3252,19 @@ found:
> return visible;
> }
>
> +bool mnt_may_suid(struct vfsmount *mnt)
> +{
> + /*
> + * Foreign mounts (accessed via fchdir or through /proc
> + * symlinks) are always treated as if they are nosuid. This
> + * prevents namespaces from trusting potentially unsafe
> + * suid/sgid bits, file caps, or security labels that originate
> + * in other namespaces.
> + */
> + return real_mount(mnt)->mnt_ns == current->nsproxy->mnt_ns &&
> + !(mnt->mnt_flags & MNT_NOSUID);

Maybe check_mnt() from fs/namespace.c can be exported and used here,
instead of open coding it.

> +}
> +
> static struct ns_common *mntns_get(struct task_struct *task)
> {
> struct ns_common *ns = NULL;
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index f822c3c11377..54a594d49733 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -81,6 +81,7 @@ extern void mntput(struct vfsmount *mnt);
> extern struct vfsmount *mntget(struct vfsmount *mnt);
> extern struct vfsmount *mnt_clone_internal(struct path *path);
> extern int __mnt_is_readonly(struct vfsmount *mnt);
> +extern bool mnt_may_suid(struct vfsmount *mnt);
>
> struct path;
> extern struct vfsmount *clone_private_mount(struct path *path);
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 175ab497e810..858d86a1b73c 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -437,7 +437,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
> if (!file_caps_enabled)
> return 0;
>
> - if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> + if (!mnt_may_suid(bprm->file->f_path.mnt))
> return 0;
> if (!in_userns(current_user_ns(), bprm->file->f_path.mnt->mnt_sb->s_user_ns))
> return 0;
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 564079c5c49d..459e71ddbc9d 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2137,7 +2137,7 @@ static int check_nnp_nosuid(const struct linux_binprm *bprm,
> const struct task_security_struct *new_tsec)
> {
> int nnp = (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS);
> - int nosuid = (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID);
> + int nosuid = !mnt_may_suid(bprm->file->f_path.mnt);
> int rc;
>
> if (!nnp && !nosuid)
>