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Re: [PATCH v2 2/2] x86: Add Kconfig option to require NX bit support
- To: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 19 Jun 2023 10:15:51 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 19 Jun 2023 08:16:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15.06.2023 17:31, Alejandro Vallejo wrote:
> This option hardens Xen by forcing it to write secure (NX-enhanced) PTEs
> regardless of the runtime NX feature bit in boot_cpu_data. This prevents an
> attacker with partial write support from affecting Xen's PTE generation
> logic by overriding the NX feature flag. The patch asserts support for the
> NX bit in PTEs at boot time and if so short-circuits the cpu_has_nx macro
> to 1.
>
> It has the nice benefit of replacing many instances of runtime checks with
> folded constants. This has several knock-on effects that improve codegen,
> saving 2.5KiB off the text section.
>
> The config option defaults to OFF for compatibility with previous
> behaviour.
>
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
Just one nit on top of Andrew's comments:
> @@ -697,9 +708,11 @@ trampoline_setup:
> jnc .Lno_nx_bit
>
> .Lhas_nx_bit:
> +#ifndef CONFIG_REQUIRE_NX
> /* Adjust EFER is NX is present */
> orb $EFER_NXE >> 8, 1 + sym_esi(trampoline_efer)
> .Lno_nx_bit:
> +#endif
In the comment the first "is" likely was meant to be "if".
Jan
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