On Tue, Jun 5, 2012 at 12:33 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > Hi, > > As-is, this could probably live in > Documentation/security/no-new-privs.txt (maybe with some examples > added). Or Documentation/prctl/no-new-privs.txt Just a decision between what it does and how you get to it, but I'd think either would make sense! > As for a manpage section, I think Michael Kerrisk would happily add a > section for PR_[SG]ET_NO_NEW_PRIVS to prctl if this could be > summarized into a paragraph or two. > > (And this reminds me I should send an update for the seccomp section > in the prctl manpage too.) Faster on the draw than me - thanks! > > On Tue, Jun 5, 2012 at 10:04 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> Hi all- >> >> As promised (although belatedly), I wrote up some proposed documentation >> for the no_new_privs feature. What should I do with it? I don't speak >> groff/troff/whatever man pages are written in. >> >> I would be happy to license this text appropriately for whatever tree >> it might end up in. In the mean time, it's GPLv2+. >> >> --- cut here --- >> >> 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. >> >> --- cut here --- >> >> --Andy > > > > -- > Kees Cook > Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
