Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / prctl / no_new_privs.txt

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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
+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.