linux-kernel - Re: [PATCH 1/6] PKEY: Introduce PKEY_ENFORCE

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Message-ID: <CALmYWFszHvHa2_DqMPZcWkVqhAtKxrbpJ4Mzg+VWLnPHQD_fsw@mail.gmail.com>
Date:   Tue, 16 May 2023 16:55:28 -0700
From:   Jeff Xu <jeffxu@...gle.com>
To:     Dave Hansen <dave.hansen@...el.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 1/6] PKEY: Introduce PKEY_ENFORCE_API flag

On Tue, May 16, 2023 at 4:14 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 5/15/23 06:05, jeffxu@...omium.org wrote:
> > --- a/arch/x86/mm/pkeys.c
> > +++ b/arch/x86/mm/pkeys.c
> > @@ -20,7 +20,7 @@ int __execute_only_pkey(struct mm_struct *mm)
> >       /* Do we need to assign a pkey for mm's execute-only maps? */
> >       if (execute_only_pkey == -1) {
> >               /* Go allocate one to use, which might fail */
> > -             execute_only_pkey = mm_pkey_alloc(mm);
> > +             execute_only_pkey = mm_pkey_alloc(mm, 0);
> >               if (execute_only_pkey < 0)
> >                       return -1;
> >               need_to_set_mm_pkey = true;
>
> In your threat model, what mechanism prevents the attacker from
> modifying executable mappings?
>
> I was trying to figure out if the implicit execute-only pkey should have
> the PKEY_ENFORCE_API bit set.  I think that in particular would probably
> cause some kind of ABI breakage, but it still reminded me that I have an
> incomplete picture of the threat model.
Yes. The main reason for not adding it now is the ABI breakage.
As a next step,  we could potentially develop mseal(), which fits more
to the code segment.
The PKEY_ENFORCE_API allows munmap(), so the user case is slightly different.

I will leave the threat model / V8 specific question to Stephan.