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>
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 8297e5b..9217169 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -72,6 +72,7 @@
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 @@
{
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 9bafc21..68f5942 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -938,6 +938,20 @@
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 e7fadde..e109e6d 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -455,6 +455,8 @@
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) {
