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Date: Thu, 12 Apr 2012 15:19:19 -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,
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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,
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markus@...omium.org, coreyb@...ux.vnet.ibm.com, jmorris@...ei.org
Subject: Re: [PATCH v18 08/15] seccomp: add system call filtering using BPF
On Thu, Apr 12, 2012 at 2:47 PM, Will Drewry <wad@...omium.org> wrote:
> This patch adds support for seccomp mode 2. Mode 2 introduces the
> ability for unprivileged processes to install system call filtering
> policy expressed in terms of a Berkeley Packet Filter (BPF) program.
> This program will be evaluated in the kernel for each system call
> the task makes and computes a result based on data in the format
> of struct seccomp_data.
>
> A filter program may be installed by calling:
> struct sock_fprog fprog = { ... };
> ...
> prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &fprog);
>
> The return value of the filter program determines if the system call is
> allowed to proceed or denied. If the first filter program installed
> allows prctl(2) calls, then the above call may be made repeatedly
> by a task to further reduce its access to the kernel. All attached
> programs must be evaluated before a system call will be allowed to
> proceed.
>
> Filter programs will be inherited across fork/clone and execve.
> However, if the task attaching the filter is unprivileged
> (!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task. This
> ensures that unprivileged tasks cannot attach filters that affect
> privileged tasks (e.g., setuid binary).
>
> There are a number of benefits to this approach. A few of which are
> as follows:
> - BPF has been exposed to userland for a long time
> - BPF optimization (and JIT'ing) are well understood
> - Userland already knows its ABI: system call numbers and desired
> arguments
> - No time-of-check-time-of-use vulnerable data accesses are possible.
> - system call arguments are loaded on access only to minimize copying
> required for system call policy decisions.
>
> Mode 2 support is restricted to architectures that enable
> HAVE_ARCH_SECCOMP_FILTER. In this patch, the primary dependency is on
> syscall_get_arguments(). The full desired scope of this feature will
> add a few minor additional requirements expressed later in this series.
> Based on discussion, SECCOMP_RET_ERRNO and SECCOMP_RET_TRACE seem to be
> the desired additional functionality.
>
> No architectures are enabled in this patch.
>
> Signed-off-by: Will Drewry <wad@...omium.org>
> Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
> Reviewed-by: Indan Zupancic <indan@....nu>
> Acked-by: Eric Paris <eparis@...hat.com>
Acked-by: Kees Cook <keescook@...omium.org>
--
Kees Cook
ChromeOS Security
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