Re: seccomp: add "seccomp" syscall

From: Kees Cook
Date: Tue Aug 12 2014 - 12:25:49 EST

On Mon, Aug 11, 2014 at 1:07 PM, Geert Uytterhoeven
<geert@xxxxxxxxxxxxxx> wrote:
> Hi Kees,
> v3.17 is gonna get a lot of new syscalls...

4 so far! :P

> On Wed, Aug 6, 2014 at 6:27 PM, Linux Kernel Mailing List
> <linux-kernel@xxxxxxxxxxxxxxx> wrote:
>> Gitweb:;a=commit;h=48dc92b9fc3926844257316e75ba11eb5c742b2c
>> Commit: 48dc92b9fc3926844257316e75ba11eb5c742b2c
>> Parent: 3b23dd12846215eff4afb073366b80c0c4d7543e
>> Refname: refs/heads/master
>> Author: Kees Cook <keescook@xxxxxxxxxxxx>
>> AuthorDate: Wed Jun 25 16:08:24 2014 -0700
>> Committer: Kees Cook <keescook@xxxxxxxxxxxx>
>> CommitDate: Fri Jul 18 12:13:37 2014 -0700
>> seccomp: add "seccomp" syscall
>> This adds the new "seccomp" syscall with both an "operation" and "flags"
>> parameter for future expansion. The third argument is a pointer value,
>> used with the SECCOMP_SET_MODE_FILTER operation. Currently, flags must
>> be 0. This is functionally equivalent to prctl(PR_SET_SECCOMP, ...).
>> In addition to the TSYNC flag later in this patch series, there is a
>> non-zero chance that this syscall could be used for configuring a fixed
>> argument area for seccomp-tracer-aware processes to pass syscall arguments
>> in the future. Hence, the use of "seccomp" not simply "seccomp_add_filter"
>> for this syscall. Additionally, this syscall uses operation, flags,
>> and user pointer for arguments because strictly passing arguments via
>> a user pointer would mean seccomp itself would be unable to trivially
>> filter the seccomp syscall itself.
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Is this something that I should enable?
> As it depends on CONFIG_SECCOMP, it only makes sense on architectures that
> already support CONFIG_SECCOMP, right?
> Does it make sense to reserve a syscall slot for it on architectures that
> don't support it yet?

I don't see a good reason to reserve the syscall slot if seccomp
filter is not already implemented. I've CCed linux-api in case there
is something I'm not considering, though.

FWIW, if someone wants to add it for m68k, the steps to support
seccomp are listed in the arch/Kconfig for HAVE_ARCH_SECCOMP_FILTER.


Kees Cook
Chrome OS Security
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