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From: Markus Gutschke <markus@chromium.org>
Date: Thu, 16 Feb 2012 13:28:49 -0800
Message-ID: <CAE6n16kweRehdozft0tKOB1P5LtGFJDCLQP=2EXfm8P0enyH=A@mail.gmail.com>
Subject: Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Will Drewry <wad@chromium.org>, linux-kernel@vger.kernel.org,
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On Thu, Feb 16, 2012 at 13:17, H. Peter Anvin <hpa@zytor.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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