Re: [PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

[Eric W. Biederman]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

> If a process gets access to a mount from a descendent or unrelated
> user namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem.
>
> 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 presented this is complete and total nonsense.  Mount propgation
strongly weakens if not completely breaks the assumptions you are making
in this code.

To write any generic code that knows anything we need to capture a user
namespace on struct super.

Further I think all we really want is to filter out security labels from
unprivileged mounts.   uids/gids and the like should be completely fine
because of the uid mappings.  

Having been down the route of comparing uids as userns uid tuples I am
convinced that anything requires us to take the user namespace into
account on a routine basis in the core will simply be broken for someone
forgetting somewhere.  This looks like a design that has that kind of
susceptibility.

> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>
> Seth, this should address a problem that's related to yours.  If a
> userns creates and untrusted fs (by any means, although admittedly fuse
> and user namespaces don't work all that well together right now), then
> this prevents shenanigans that could happen when the userns passes an fd
> pointing at the filesystem out to the root ns.

Andy for now I really think we are best not even reading those
capabilities into the vfs from unprivileged mounts.

Eric

>  fs/exec.c                |  2 +-
>  fs/namespace.c           | 21 +++++++++++++++++++++
>  include/linux/mount.h    |  1 +
>  security/commoncap.c     |  2 +-
>  security/selinux/hooks.c |  4 ++--
>  5 files changed, 26 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 a01c7730e9af..53301680ea7e 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3011,6 +3011,27 @@ found:
>  	return visible;
>  }
>  
> +bool mnt_may_suid(struct vfsmount *mnt)
> +{
> +	struct user_namespace *mount_userns = real_mount(mnt)->mnt_ns->user_ns;
> +	struct user_namespace *ns;
> +
> +	if (mnt->mnt_flags & MNT_NOSUID)
> +		return false;
> +
> +	/*
> +	 * We only trust mounts in our own namespace or its parents; we
> +	 * treat untrusted mounts as MNT_NOSUID regardless of whether
> +	 * they have MNT_NOSUID set.
> +	 */
> +	for (ns = current_user_ns(); ns; ns = ns->parent) {
> +		if (ns == mount_userns)
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
>  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;
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/