Re: [PATCH v3] fs: Treat foreign mounts as nosuid

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting 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>

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

> ---
> 
> Changes from v2:
>  - Fix comment typo.
> 
> Changes from v1:
>  - Treat all foreign mounts as nosuid, not just non-self-or-ancestor
>    userns mounts.
> 
>  fs/exec.c                |  2 +-
>  fs/namespace.c           | 13 +++++++++++++
>  include/linux/mount.h    |  1 +
>  security/commoncap.c     |  2 +-
>  security/selinux/hooks.c |  4 ++--
>  5 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index a2b42a98c743..ac0bb22aa3ed 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1267,7 +1267,7 @@ int prepare_binprm(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) &&
>  	    !task_no_new_privs(current) &&
>  	    kuid_has_mapping(bprm->cred->user_ns, inode->i_uid) &&
>  	    kgid_has_mapping(bprm->cred->user_ns, inode->i_gid)) {
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ef42d9bee212..4df0b393c29d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3019,6 +3019,19 @@ 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);
> +}
> +
>  static void *mntns_get(struct task_struct *task)
>  {
>  	struct mnt_namespace *ns = NULL;
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 9262e4bf0cc3..b7b84bafe09b 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -80,6 +80,7 @@ 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 file_system_type;
>  extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
> diff --git a/security/commoncap.c b/security/commoncap.c
> index bab0611afc1e..52b3eed065e0 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -443,7 +443,7 @@ 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;
>  
>  	dentry = dget(bprm->file->f_dentry);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index b0e940497e23..2089fd0d539e 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2139,7 +2139,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
>  		 */
>  		if (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)
>  			return -EPERM;
> -		if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> +		if (!mnt_may_suid(bprm->file->f_path.mnt))
>  			return -EACCES;
>  	} else {
>  		/* Check for a default transition on this program. */
> @@ -2153,7 +2153,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
>  	ad.type = LSM_AUDIT_DATA_PATH;
>  	ad.u.path = bprm->file->f_path;
>  
> -	if ((bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
> +	if (!mnt_may_suid(bprm->file->f_path.mnt) ||
>  	    (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS))
>  		new_tsec->sid = old_tsec->sid;
>  
> -- 
> 1.9.3
> 
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