linux-kernel - Re: [PATCH] x86/mm: Disable W^X detection and enforcement on 32-bit

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Message-ID: <CAMj1kXEAneXVTFenwrP5U6-SSO7Hqm07z=1w19PUS4MUHM+BRw@mail.gmail.com>
Date:   Sat, 24 Sep 2022 09:26:48 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-efi@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/mm: Disable W^X detection and enforcement on 32-bit

On Sat, 24 Sept 2022 at 00:17, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>
> The 32-bit code is in a weird spot.  Some 32-bit builds (non-PAE) do not
> even have NX support.  Even PAE builds that support NX have to contend
> with things like EFI data and code mixed in the same pages where W+X
> is unavoidable.
>
> The folks still running X86_32=y kernels are unlikely to care much about
> NX.  That combined with the fundamental inability fix _all_ of the W+X
> things means this code had little value on X86_32=y.  Disable the checks.
>
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Darren Hart <dvhart@...radead.org>
> Cc: Andy Shevchenko <andy@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: x86@...nel.org
> Cc: linux-efi@...r.kernel.org
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Kees Cook <keescook@...omium.org>
> Link: https://lore.kernel.org/all/CAMj1kXHcF_iK_g0OZSkSv56Wmr=eQGQwNstcNjLEfS=mm7a06w@mail.gmail.com/

Acked-by: Ard Biesheuvel <ardb@...nel.org>

> ---
>  arch/x86/mm/pat/set_memory.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 20b1e24baa85..efe882c753ca 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -587,6 +587,14 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
>  {
>         unsigned long end;
>
> +       /*
> +        * 32-bit has some unfixable W+X issues, like EFI code
> +        * and writeable data being in the same page.  Disable
> +        * detection and enforcement there.
> +        */
> +       if (IS_ENABLED(CONFIG_X86_32))
> +               return new;
> +
>         /* Only enforce when NX is supported: */
>         if (!(__supported_pte_mask & _PAGE_NX))
>                 return new;
> --
> 2.34.1
>