fs: Limit file caps to the user namespace of the super block

Capability sets attached to files must be ignored except in the
user namespaces where the mounter is privileged, i.e. s_user_ns
and its descendants. Otherwise a vector exists for gaining
privileges in namespaces where a user is not already privileged.

Add a new helper function, current_in_user_ns(), to test whether a user
namespace is the same as or a descendant of another namespace.
Use this helper to determine whether a file's capability set
should be applied to the caps constructed during exec.

--EWB Replaced in_userns with the simpler current_in_userns.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

3 files changed, 22 insertions, 0 deletions

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 8297e5b341d8..9217169c64cb 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -72,6 +72,7 @@ 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 current_in_userns(const struct user_namespace *target_ns);
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
@@ -100,6 +101,11 @@ static inline bool userns_may_setgroups(const struct user_namespace *ns)
 {
         return true;
 }
+
+static inline bool current_in_userns(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 9bafc211930c..68f594212759 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -938,6 +938,20 @@ 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 current_in_userns(const struct user_namespace *target_ns)
+{
+        struct user_namespace *ns;
+        for (ns = current_user_ns(); ns; ns = ns->parent) {
+                if (ns == target_ns)
+                        return true;
+        }
+        return false;
+}
+
 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 e7fadde737f4..e109e6dac858 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -455,6 +455,8 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective, bool *has_c
 
         if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
                 return 0;
+        if (!current_in_userns(bprm->file->f_path.mnt->mnt_sb->s_user_ns))
+                return 0;
 
         rc = get_vfs_caps_from_disk(bprm->file->f_path.dentry, &vcaps);
         if (rc < 0) {