[RFC v2 PATCH 3/8] fs: Treat foreign mounts as nosuid

[Djalal Harouni <tixxdz@gmail.com>]

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 patch was just adapted from the original one that was written
by Andy Lutomirski <luto@amacapital.net>
https://www.redhat.com/archives/dm-devel/2016-April/msg00374.html

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
---
 fs/exec.c                      |  2 +-
 fs/namespace.c                 | 15 +++++++++++++++
 include/linux/mount.h          |  1 +
 include/linux/user_namespace.h |  8 ++++++++
 kernel/user_namespace.c        | 13 +++++++++++++
 security/commoncap.c           |  2 +-
 security/selinux/hooks.c       |  2 +-
 7 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index c4010b8..706088d 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1391,7 +1391,7 @@ 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 de02b39..a8820fb 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3374,6 +3374,21 @@ found:
 	return visible;
 }
 
+bool mnt_may_suid(struct vfsmount *mnt)
+{
+	struct mount *m = real_mount(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(m) &&
+		 in_userns(current_user_ns(), m->mnt_ns->user_ns);
+}
+
 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 f822c3c..54a594d 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/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 8297e5b..a43faa7 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -72,6 +72,8 @@ extern ssize_t proc_projid_map_write(struct file *, const char __user *, size_t,
 extern ssize_t proc_setgroups_write(struct file *, const char __user *, size_t, loff_t *);
 extern int proc_setgroups_show(struct seq_file *m, void *v);
 extern bool userns_may_setgroups(const struct user_namespace *ns);
+extern bool in_userns(const struct user_namespace *ns,
+		      const struct user_namespace *target_ns);
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
@@ -100,6 +102,12 @@ static inline bool userns_may_setgroups(const struct user_namespace *ns)
 {
 	return true;
 }
+
+static inline bool in_userns(const struct user_namespace *ns,
+			     const struct user_namespace *target_ns)
+{
+	return true;
+}
 #endif
 
 #endif /* _LINUX_USER_H */
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 9bafc21..9a496a8 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -938,6 +938,19 @@ bool userns_may_setgroups(const struct user_namespace *ns)
 	return allowed;
 }
 
+/*
+ * Returns true if @ns is the same namespace as or a descendant of
+ * @target_ns.
+ */
+bool in_userns(const struct user_namespace *ns,
+	       const struct user_namespace *target_ns)
+{
+	for (; ns; ns = ns->parent) {
+		if (ns == target_ns)
+			return true;
+	}
+}
+
 static inline struct user_namespace *to_user_ns(struct ns_common *ns)
 {
 	return container_of(ns, struct user_namespace, ns);
diff --git a/security/commoncap.c b/security/commoncap.c
index 48071ed..6c082d2 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -453,7 +453,7 @@ 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;
 
 	rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 912deee..1350167 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2234,7 +2234,7 @@ 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)
-- 
2.5.5