doc: ReSTify no_new_privs.txt

This updates no_new_privs documentation to ReST markup and adds it to
the user-space API documentation.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 64 insertions, 0 deletions

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index 15ff12342db8..7b2eb1b7d4ca 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -16,6 +16,7 @@ place where this information is gathered.
 .. toctree::
    :maxdepth: 2
 
+   no_new_privs
    seccomp_filter
    unshare
 
diff --git a/Documentation/userspace-api/no_new_privs.rst b/Documentation/userspace-api/no_new_privs.rst
new file mode 100644
index 000000000000..d060ea217ea1
--- /dev/null
+++ b/Documentation/userspace-api/no_new_privs.rst
@@ -0,0 +1,63 @@
+======================
+No New Privileges Flag
+======================
+
+The execve system call can grant a newly-started program privileges that
+its parent did not have.  The most obvious examples are setuid/setgid
+programs and file capabilities.  To prevent the parent program from
+gaining these privileges as well, the kernel and user code must be
+careful to prevent the parent from doing anything that could subvert the
+child.  For example:
+
+ - The dynamic loader handles ``LD_*`` environment variables differently if
+   a program is setuid.
+
+ - chroot is disallowed to unprivileged processes, since it would allow
+   ``/etc/passwd`` to be replaced from the point of view of a process that
+   inherited chroot.
+
+ - The exec code has special handling for ptrace.
+
+These are all ad-hoc fixes.  The ``no_new_privs`` bit (since Linux 3.5) is a
+new, generic mechanism to make it safe for a process to modify its
+execution environment in a manner that persists across execve.  Any task
+can set ``no_new_privs``.  Once the bit is set, it is inherited across fork,
+clone, and execve and cannot be unset.  With ``no_new_privs`` set, ``execve()``
+promises not to grant the privilege to do anything that could not have
+been done without the execve call.  For example, the setuid and setgid
+bits will no longer change the uid or gid; file capabilities will not
+add to the permitted set, and LSMs will not relax constraints after
+execve.
+
+To set ``no_new_privs``, use::
+
+    prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+
+Be careful, though: LSMs might also not tighten constraints on exec
+in ``no_new_privs`` mode.  (This means that setting up a general-purpose
+service launcher to set ``no_new_privs`` before execing daemons may
+interfere with LSM-based sandboxing.)
+
+Note that ``no_new_privs`` does not prevent privilege changes that do not
+involve ``execve()``.  An appropriately privileged task can still call
+``setuid(2)`` and receive SCM_RIGHTS datagrams.
+
+There are two main use cases for ``no_new_privs`` so far:
+
+ - Filters installed for the seccomp mode 2 sandbox persist across
+   execve and can change the behavior of newly-executed programs.
+   Unprivileged users are therefore only allowed to install such filters
+   if ``no_new_privs`` is set.
+
+ - By itself, ``no_new_privs`` can be used to reduce the attack surface
+   available to an unprivileged user.  If everything running with a
+   given uid has ``no_new_privs`` set, then that uid will be unable to
+   escalate its privileges by directly attacking setuid, setgid, and
+   fcap-using binaries; it will need to compromise something without the
+   ``no_new_privs`` bit set first.
+
+In the future, other potentially dangerous kernel features could become
+available to unprivileged tasks if ``no_new_privs`` is set.  In principle,
+several options to ``unshare(2)`` and ``clone(2)`` would be safe when
+``no_new_privs`` is set, and ``no_new_privs`` + ``chroot`` is considerable less
+dangerous than chroot by itself.