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authorAndy Lutomirski <luto@amacapital.net>2012-07-02 14:03:58 -0700
committerJames Morris <james.l.morris@oracle.com>2012-07-03 12:35:36 +1000
commit09b243577be319ef55310b45c65737008f3ebf12 (patch)
tree855f48235a5b6288e9ff17b97fc338341320e542 /Documentation/prctl
parentca24a145573124732152daff105ba68cc9a2b545 (diff)
downloadlinux-09b243577be319ef55310b45c65737008f3ebf12.tar.gz
security: document no_new_privs
Document no_new_privs. Signed-off-by: Andy Lutomirski <luto@amacapital.net> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: James Morris <james.l.morris@oracle.com>
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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.
+
+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.