[v4] x86_64: Improvements at compressed kernel stage

[PATCH v4 03/26] x86/boot: Set cr0 to known state in trampoline

Posted by Evgeniy Baskov 2 years, 9 months ago

Ensure WP bit to be set to prevent boot code from writing to
non-writable memory pages.

Tested-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Evgeniy Baskov <baskov@ispras.ru>
---
 arch/x86/boot/compressed/head_64.S | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index a75712991df3..9f2e8f50fc71 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src)
 	pushl	$__KERNEL_CS
 	pushl	%eax
 
-	/* Enable paging again. */
-	movl	%cr0, %eax
-	btsl	$X86_CR0_PG_BIT, %eax
+	/* Enable paging and set CR0 to known state (this also sets WP flag) */
+	movl	$CR0_STATE, %eax
 	movl	%eax, %cr0
 
 	lret
-- 
2.37.4

Re: [PATCH v4 03/26] x86/boot: Set cr0 to known state in trampoline

Posted by Ard Biesheuvel 2 years, 6 months ago

On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@ispras.ru> wrote:
>
> Ensure WP bit to be set to prevent boot code from writing to
> non-writable memory pages.
>
> Tested-by: Mario Limonciello <mario.limonciello@amd.com>
> Tested-by: Peter Jones <pjones@redhat.com>
> Signed-off-by: Evgeniy Baskov <baskov@ispras.ru>

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

> ---
>  arch/x86/boot/compressed/head_64.S | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index a75712991df3..9f2e8f50fc71 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src)
>         pushl   $__KERNEL_CS
>         pushl   %eax
>
> -       /* Enable paging again. */
> -       movl    %cr0, %eax
> -       btsl    $X86_CR0_PG_BIT, %eax
> +       /* Enable paging and set CR0 to known state (this also sets WP flag) */
> +       movl    $CR0_STATE, %eax
>         movl    %eax, %cr0
>
>         lret
> --
> 2.37.4
>