netdev - Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

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Date:	Thu, 16 Feb 2012 13:28:49 -0800
From:	Markus Gutschke <markus@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net,
	mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
	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, keescook@...omium.org
Subject: Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF

On Thu, Feb 16, 2012 at 13:17, H. Peter Anvin <hpa@...or.com> wrote:
> The other thing that you really need in addition to system call number is
> ABI identifier, since a syscall number may mean different things for
> different entry points.  For example, on x86-64 system call number 4 is
> write() if called via int $0x80 but stat() if called via syscall64. This is
> a local property of the system call, not a global per process.

I think, the documentation said that as soon as prctl() is used to set
a bpf filter for system calls, it automatically disallows system calls
using an entry point other than the one used by this particular
prctl().

I was trying to come up with scenarios where this particular approach
causes problem, but I can't think of any off the top of my head. So,
it might actually turn out to be a very elegant way to reduce the
attack surface of the kernel. If we are really worried about userspace
compatibility, we could make the kernel send a signal instead of
terminating the program, if the wrong entry point was used; not sure
if that is needed, though.

Markus
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