security: document no_new_privs

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.

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.