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

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Sep 23, 2022 at 03:17:30PM -0700, Dave Hansen 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@roeck-us.net>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Darren Hart <dvhart@infradead.org>
> Cc: Andy Shevchenko <andy@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: x86@kernel.org
> Cc: linux-efi@vger.kernel.org
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/all/CAMj1kXHcF_iK_g0OZSkSv56Wmr=eQGQwNstcNjLEfS=mm7a06w@mail.gmail.com/

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> ---
>  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
>