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

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Date:   Tue, 16 May 2023 16:14:47 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     jeffxu@...omium.org, luto@...nel.org, jorgelo@...omium.org,
        keescook@...omium.org, groeck@...omium.org, jannh@...gle.com,
        sroettger@...gle.com
Cc:     akpm@...ux-foundation.org, jeffxu@...gle.com,
        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 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.