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+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
+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.