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