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Date: Thu, 12 Apr 2012 15:17:01 -0700
From: Kees Cook <keescook@...omium.org>
To: Will Drewry <wad@...omium.org>
Cc: linux-kernel@...r.kernel.org, linux-man@...r.kernel.org,
linux-security-module@...r.kernel.org, linux-arch@...r.kernel.org,
linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
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rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
scarybeasts@...il.com, indan@....nu, pmoore@...hat.com,
akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
markus@...omium.org, coreyb@...ux.vnet.ibm.com, jmorris@...ei.org,
Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v18 01/15] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve
from granting privs
On Thu, Apr 12, 2012 at 2:47 PM, Will Drewry <wad@...omium.org> wrote:
> From: Andy Lutomirski <luto@...capital.net>
>
> With this change, calling
> prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)
> disables privilege granting operations at execve-time. For example, a
> process will not be able to execute a setuid binary to change their uid
> or gid if this bit is set. The same is true for file capabilities.
>
> Additionally, LSM_UNSAFE_NO_NEW_PRIVS is defined to ensure that
> LSMs respect the requested behavior.
>
> To determine if the NO_NEW_PRIVS bit is set, a task may call
> prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
> It returns 1 if set and 0 if it is not set. If any of the arguments are
> non-zero, it will return -1 and set errno to -EINVAL.
> (PR_SET_NO_NEW_PRIVS behaves similarly.)
>
> This functionality is desired for the proposed seccomp filter patch
> series. By using PR_SET_NO_NEW_PRIVS, it allows a task to modify the
> system call behavior for itself and its child tasks without being
> able to impact the behavior of a more privileged task.
>
> Another potential use is making certain privileged operations
> unprivileged. For example, chroot may be considered "safe" if it cannot
> affect privileged tasks.
>
> Note, this patch causes execve to fail when PR_SET_NO_NEW_PRIVS is
> set and AppArmor is in use. It is fixed in a subsequent patch.
>
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Will Drewry <wad@...omium.org>
> Acked-by: Eric Paris <eparis@...hat.com>
Acked-by: Kees Cook <keescook@...omium.org>
--
Kees Cook
ChromeOS Security
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