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Subject: Re: For review: seccomp_user_notif(2) manual page
To:     Kees Cook <keescook@chromium.org>
References: <45f07f17-18b6-d187-0914-6f341fe90857@gmail.com>
 <202009301632.9C6A850272@keescook>
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <c1cc0df8-bd06-51e7-d5a0-888c1955683b@gmail.com>
Date:   Thu, 15 Oct 2020 13:24:03 +0200
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Hello Kees,

On 10/1/20 1:39 AM, Kees Cook wrote:
> On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
>> [...] I did :-)
> 
> Yay! Thank you!

You're welcome :-)

>> [...]
>>    Overview
>>        In conventional usage of a seccomp filter, the decision about how
>>        to  treat  a particular system call is made by the filter itself.
>>        The user-space notification mechanism allows the handling of  the
>>        system  call  to  instead  be handed off to a user-space process.
>>        The advantages of doing this are that, by contrast with the  sec‐
>>        comp  filter,  which  is  running on a virtual machine inside the
>>        kernel, the user-space process has access to information that  is
>>        unavailable to the seccomp filter and it can perform actions that
>>        can't be performed from the seccomp filter.
> 
> I might clarify a bit with something like (though maybe the
> target/supervisor paragraph needs to be moved to the start):
> 
> 	This is used for performing syscalls on behalf of the target,
> 	rather than having the supervisor make security policy decisions
> 	about the syscall, which would be inherently race-prone. The
> 	target's syscall should either be handled by the supervisor or
> 	allowed to continue normally in the kernel (where standard security
> 	policies will be applied).

You, Christian, and Jann all pulled me up on this point. And thanks; 
I'm going to use some of your words above. See my reply to Jann, sent
at about the same time as this reply. Please take a look at the text
in my reply to Jann, and let me know what you think.

> I'll comment more later, but I've run out of time today and I didn't see
> anyone mention this detail yet in the existing threads... :)

Later never came :-). But, I hope you may have comments for the 
next draft, which I will send out soon.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/