Date: Fri, 16 Feb 2024 19:24:50 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Jonathan Corbet <corbet@....net>, Petr Tesařík
 <petr@...arici.cz>, Dave Hansen <dave.hansen@...el.com>
Cc: Petr Tesarik <petrtesarik@...weicloud.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Xin Li <xin3.li@...el.com>, Arnd Bergmann <arnd@...db.de>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Kees Cook <keescook@...omium.org>,
 "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
 Pengfei Xu <pengfei.xu@...el.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Ze Gao <zegao2021@...il.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Brian Gerst <brgerst@...il.com>, Jason Gunthorpe <jgg@...pe.ca>,
 Joerg Roedel <jroedel@...e.de>, "Mike Rapoport (IBM)" <rppt@...nel.org>,
 Tina Zhang <tina.zhang@...el.com>, Jacob Pan
 <jacob.jun.pan@...ux.intel.com>,
 "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>, David Howells
 <dhowells@...hat.com>, Petr Tesarik <petr.tesarik1@...wei-partners.com>,
 jannh@...gle.com, linux-security-module@...r.kernel.org
Subject: Re: [RFC 0/8] PGP key parser using SandBox Mode

On 2/16/2024 6:21 PM, Jonathan Corbet wrote:
> Petr Tesařík <petr@...arici.cz> writes:
> 
>> On Fri, 16 Feb 2024 07:38:30 -0800
>> Dave Hansen <dave.hansen@...el.com> wrote:
>>> I'm confused by this.  The kernel doesn't (appear to) have a PGP parser
>>> today.  So are you saying that it *should* have one and it's only
>>> feasible if its confined in a sandbox?
>>
>> I'm sorry if this is confusing. Yes, your understanding is correct.
>> This patch series demonstrates that SBM (even in the initial version
>> that was submitted) allows to write a PGP parser which can survive
>> memory safety bugs withoug compromising the rest of the kernel.
> 
> So I have a different question: some years ago we added the "usermode
> blob" feature for just this kind of use case - parsing firewall rules at
> the time.  It has never been used for that, but it's still there in
> kernel/usermode_driver.c.  Is there a reason why this existing
> functionality can't be used for tasks like PGP parsing as well?

Yes, it was an option I explored last year (briefly talked about it as a 
BoF at LSS NA 2023).

You are right, there is such feature that seemed to fit well.

User space blob embedded in a kernel module, so signed. User space 
process connected only to the kernel through a pipe.

I even went ahead, and created a framework:

https://lore.kernel.org/linux-kernel/20230317145240.363908-1-roberto.sassu@huaweicloud.com/

so that anyone can implement similar use cases.

The further step is: how can I ensure that the process launched by the 
kernel is not attacked by root (which I assumed to be less powerful than 
the kernel in a locked-down system).

I handled this in both directions:

- The process launched by the kernel is under a seccomp strict profile,
   and can only read/write through the pipe created by the kernel (and
   call few other system calls, such as exit()). Otherwise it is killed.
   Cannot create any new communication channel.

- I created an LSM that denies any attempt to ptrace/signal to the
   process launched by the kernel. Jann Horn also suggested to make the
   process non-swappable.

However, despite these attempts, security people don't feel confident on 
offloading a kernel workload outside the kernel.

This is why this work started.

Roberto

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