[Devel] [PATCH RH7 11/22] port/fs: Treat foreign mounts as nosuid

[Pavel Tikhomirov]

change: move hunk for check_nnp_nosuid to selinux_bprm_set_creds

If a process gets access to a mount from a different user
namespace, that process should not be able to take advantage of
setuid files or selinux entrypoints from that filesystem.  Prevent
this by treating mounts from other mount namespaces and those not
owned by current_user_ns() or an ancestor as nosuid.

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 at amacapital.net>
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Acked-by: James Morris <james.l.morris at oracle.com>
Acked-by: Serge Hallyn <serge.hallyn at canonical.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.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 bba8220..a62f814 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1284,7 +1284,7 @@ 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 (current->no_new_privs)
diff --git a/fs/namespace.c b/fs/namespace.c
index bea4ebf..97fa6a7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3069,6 +3069,19 @@ void update_mnt_policy(struct user_namespace *userns)
 	up_read(&namespace_sem);
 }
 
+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 !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) &&
+	       in_userns(current_user_ns(), mnt->mnt_sb->s_user_ns);
+}
+
 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 034b26a..1f87105 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -84,6 +84,7 @@ extern struct vfsmount *mntget(struct vfsmount *mnt);
 extern void mnt_pin(struct vfsmount *mnt);
 extern void mnt_unpin(struct vfsmount *mnt);
 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 c313a40..ca0c04ae 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;
 	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 57a2020..6a38d9f 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2108,7 +2108,7 @@ 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