Re: [RFC 0/8] PGP key parser using SandBox Mode

From: Roberto Sassu <roberto.sassu@huaweicloud.com>
To: "Jonathan Corbet" <corbet@lwn.net>,
	"Petr Tesařík" <petr@tesarici.cz>,
	"Dave Hansen" <dave.hansen@intel.com>
Cc: Petr Tesarik <petrtesarik@huaweicloud.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>, Xin Li <xin3.li@intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Kees Cook <keescook@chromium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
	Pengfei Xu <pengfei.xu@intel.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Ze Gao <zegao2021@gmail.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Kai Huang <kai.huang@intel.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Brian Gerst <brgerst@gmail.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	Joerg Roedel <jroedel@suse.de>,
	"Mike Rapoport (IBM)" <rppt@kernel.org>,
	Tina Zhang <tina.zhang@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	David Howells <dhowells@redhat.com>,
	Petr Tesarik <petr.tesarik1@huawei-partners.com>,
	jannh@google.com, linux-security-module@vger.kernel.org
Subject: Re: [RFC 0/8] PGP key parser using SandBox Mode
Date: Fri, 16 Feb 2024 19:24:50 +0100	[thread overview]
Message-ID: <33a1fae4-d713-4e93-89ff-ff9f377e8391@huaweicloud.com> (raw)
In-Reply-To: <87y1bktjdk.fsf@meer.lwn.net>

On 2/16/2024 6:21 PM, Jonathan Corbet wrote:
> Petr Tesařík <petr@tesarici.cz> writes:
> 
>> On Fri, 16 Feb 2024 07:38:30 -0800
>> Dave Hansen <dave.hansen@intel.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