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From: Dave Hansen <dave.hansen@intel.com>
To: Jann Horn <jannh@google.com>
Cc: wad@chromium.org, Kees Cook <keescook@chromium.org>,
	Linux API <linux-api@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	linuxram@us.ibm.com, Andy Lutomirski <luto@amacapital.net>,
	msammler@mpi-sws.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH] seccomp: Add protection keys into seccomp_data
Date: Mon, 29 Oct 2018 10:29:18 -0700
Message-ID: <62e09400-0443-8db9-a389-ba4f4201226b@intel.com> (raw)
In-Reply-To: <CAG48ez1mY7Osdsc4iJ1JKq=C60wb9bQF=aHbi1fb=pb_V2XMJA@mail.gmail.com>

On 10/29/18 9:48 AM, Jann Horn wrote:
> On Mon, Oct 29, 2018 at 5:37 PM Dave Hansen <dave.hansen@intel.com> wrote:
>> I'm not sure this is a great use for PKRU.  I *think* the basic problem
>> is that you want to communicate some rights information down into a
>> filter, and you want to communicate it with PKRU.  While it's handy to
>> have an extra register that nobody (generally) mucks with, I'm not quite
>> convinced that we want to repurpose it this way.
> 
> That's not how I understand it; I believe that the context is probably
> https://arxiv.org/pdf/1801.06822.pdf ?
> My understanding is that PKRU is used for lightweight in-process
> sandboxing, and to extend this sandbox protection to the syscall
> interface, it is necessary to expose PKRU state to seccomp filters.
> In other words, this isn't using PKRU exclusively for passing rights
> into a filter, but it has to use PKRU anyway.

PKRU gives information about rights to various bits of application data.
 From that, a seccomp filter can infer the context, and thus the ability
for the code to call a given syscall at a certain point in time.

This makes PKRU an opt-in part of the syscall ABI, which is pretty
interesting.  We _could_ do the same kind of thing with any callee-saved
general purpose register, but PKRU is particularly attractive because
there is only one instruction that writes to it (well, outside of
XSAVE*), and random library code is very unlikely at this point to be
using it.

PKRU getting reset on signals, and the requirement now that it *can't*
be changed if you make syscalls probably needs to get thought about very
carefully before we do this, though.

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-29 11:23 Michael Sammler
2018-10-29 16:25 ` Kees Cook
2018-10-29 16:37   ` Dave Hansen
2018-10-29 16:48     ` Jann Horn
2018-10-29 17:02       ` Michael Sammler
2018-10-29 17:07         ` Dave Hansen
2018-10-29 17:29       ` Dave Hansen [this message]
2018-10-29 21:55         ` Michael Sammler
2018-10-29 22:33           ` Dave Hansen
2018-10-30 10:55             ` Michael Sammler
2018-10-29 16:42   ` Jann Horn
2018-10-29 16:48   ` Ram Pai
2018-10-29 17:05     ` Michael Sammler

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