linux-hardening - Re: [PATCH 2/6] PKEY: Add arch_check_pkey_enforce

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Date:   Thu, 18 May 2023 17:00:01 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jeff Xu <jeffxu@...gle.com>
Cc:     jeffxu@...omium.org, luto@...nel.org, jorgelo@...omium.org,
        keescook@...omium.org, groeck@...omium.org, jannh@...gle.com,
        sroettger@...gle.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/6] PKEY: Add arch_check_pkey_enforce_api()

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.