Re: [PATCH 2/6] PKEY: Add arch_check_pkey_enforce_api()

From: "Stephen Röttger" <sroettger@google.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Jeff Xu <jeffxu@google.com>,
	jeffxu@chromium.org, luto@kernel.org, jorgelo@chromium.org,
	keescook@chromium.org, groeck@chromium.org, jannh@google.com,
	akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-mm@kvack.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH 2/6] PKEY: Add arch_check_pkey_enforce_api()
Date: Fri, 19 May 2023 13:22:21 +0200	[thread overview]
Message-ID: <CAEAAPHb5wzn6=9sL92-wq7mwT0-iu7NVmzpWM7tSiN85kZYO9w@mail.gmail.com> (raw)
In-Reply-To: <c53c03e8-529f-5b72-42ab-f32f50aaab35@intel.com>

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On Fri, May 19, 2023 at 2:00 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 5/18/23 15:51, Jeff Xu wrote:
> >> Do you have a solid handle on all call paths that will reach
> >> __arch_check_vma_pkey_for_write() and can you ensure they are all
> >> non-remote?
> > Is this about the attack scenario where the attacker uses ptrace()
> > into the chrome process ? if so it is not in our threat model, and
> > that is more related to sandboxing on the host.
>
> The attacker would use *some* remote interface.  ptrace() is just one of
> those remote interfaces.
>
> > Or is this about io_uring? Yes, io_uring kernel thread breaks our
> > expectations of PKRU & user space threads, however I thought the break
> > is not just for this - any syscall involved in memory operation will
> > break after into io_uring ?
>
> I'm not quite following.
>
> Please just do me a favor: have the io_uring maintainers look at your
> proposal.  Make sure that the defenses you are building can work in a
> process where io_uring is in use by the benign threads.
>
> Those same folks are pretty familiar with the other, more traditional
> I/O syscalls that have in-memory descriptors that control syscall
> behavior like readv/writev.  Those also need a close look.
>
> > Other than those, yes, I try to ensure the check is only used at the
> > beginning of syscall entry in all cases, which should be non-remote I
> > hope.
>
> You're right that synchronous, shallow syscall paths are usually
> non-remote.  But those aren't the problem.  The problem is that there
> *ARE* remote accesses and those are a potential hole for this whole
> mechanism.
>
> Can they be closed?  I don't know.  I honestly don't have a great grasp
> on how widespread these things are.  You'll need a much more complete
> grasp on them than I have before this thing can go forward.

I don't think the remote writes are a problem for us if they're initiated from
the same process. It's a case of syscalls where we need to add special
validation in userspace.

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