1====================== 2No New Privileges Flag 3====================== 4 5The execve system call can grant a newly-started program privileges that 6its parent did not have. The most obvious examples are setuid/setgid 7programs and file capabilities. To prevent the parent program from 8gaining these privileges as well, the kernel and user code must be 9careful to prevent the parent from doing anything that could subvert the 10child. For example: 11 12 - The dynamic loader handles ``LD_*`` environment variables differently if 13 a program is setuid. 14 15 - chroot is disallowed to unprivileged processes, since it would allow 16 ``/etc/passwd`` to be replaced from the point of view of a process that 17 inherited chroot. 18 19 - The exec code has special handling for ptrace. 20 21These are all ad-hoc fixes. The ``no_new_privs`` bit (since Linux 3.5) is a 22new, generic mechanism to make it safe for a process to modify its 23execution environment in a manner that persists across execve. Any task 24can set ``no_new_privs``. Once the bit is set, it is inherited across fork, 25clone, and execve and cannot be unset. With ``no_new_privs`` set, ``execve()`` 26promises not to grant the privilege to do anything that could not have 27been done without the execve call. For example, the setuid and setgid 28bits will no longer change the uid or gid; file capabilities will not 29add to the permitted set, and LSMs will not relax constraints after 30execve. 31 32To set ``no_new_privs``, use:: 33 34 prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); 35 36Be careful, though: LSMs might also not tighten constraints on exec 37in ``no_new_privs`` mode. (This means that setting up a general-purpose 38service launcher to set ``no_new_privs`` before execing daemons may 39interfere with LSM-based sandboxing.) 40 41Note that ``no_new_privs`` does not prevent privilege changes that do not 42involve ``execve()``. An appropriately privileged task can still call 43``setuid(2)`` and receive SCM_RIGHTS datagrams. 44 45There are two main use cases for ``no_new_privs`` so far: 46 47 - Filters installed for the seccomp mode 2 sandbox persist across 48 execve and can change the behavior of newly-executed programs. 49 Unprivileged users are therefore only allowed to install such filters 50 if ``no_new_privs`` is set. 51 52 - By itself, ``no_new_privs`` can be used to reduce the attack surface 53 available to an unprivileged user. If everything running with a 54 given uid has ``no_new_privs`` set, then that uid will be unable to 55 escalate its privileges by directly attacking setuid, setgid, and 56 fcap-using binaries; it will need to compromise something without the 57 ``no_new_privs`` bit set first. 58 59In the future, other potentially dangerous kernel features could become 60available to unprivileged tasks if ``no_new_privs`` is set. In principle, 61several options to ``unshare(2)`` and ``clone(2)`` would be safe when 62``no_new_privs`` is set, and ``no_new_privs`` + ``chroot`` is considerable less 63dangerous than chroot by itself. 64