Re: [RFC PATCH] x86: Make sure verify_cpu has a good stack

[Yinghai Lu]

On Thu, Mar 3, 2016 at 4:28 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> From: Borislav Petkov <bp@xxxxxxx>
> Date: Sun, 28 Feb 2016 21:35:44 +0100
> Subject: [PATCH -v2] x86/asm: Make sure verify_cpu() has a good stack

> 04633df0c43d ("x86/cpu: Call verify_cpu() after having entered long mode too")
> added the call to verify_cpu() for sanitizing CPU configuration.
>
> The latter uses the stack minimally and it can happen that we land in
> startup_64() directly from a 64-bit bootloader. Then we want to use our
> own, known good stack.
>
> Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Mika Penttilà <mika.penttila@xxxxxxxxxxxx>
> ---
>  arch/x86/kernel/head_64.S         | 3 +++
>  include/asm-generic/vmlinux.lds.h | 4 +++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 22fbf9df61bb..968d6408b887 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -64,6 +64,9 @@ startup_64:
>          * tables and then reload them.
>          */
>
> +       /* Setup stack for verify_cpu(). */
> +       leaq    (__end_init_task - 8)(%rip), %rsp
> +
>         /* Sanitize CPU configuration */
>         call verify_cpu
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 772c784ba763..cba2a26628fc 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -246,7 +246,9 @@
>
>  #define INIT_TASK_DATA(align)                                          \
>         . = ALIGN(align);                                               \
> -       *(.data..init_task)
> +       VMLINUX_SYMBOL(__start_init_task) = .;                          \
> +       *(.data..init_task)                                             \
> +       VMLINUX_SYMBOL(__end_init_task) = .;
>
>  /*
>   * Read only Data
> --
> 2.3.5

I would suggest moving down verify_cpu calling after offset is
calcuated, like following instead of adding __end_init_task.

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 22fbf9d..e8c8085 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -64,9 +64,6 @@ startup_64:
         * tables and then reload them.
         */

-       /* Sanitize CPU configuration */
-       call verify_cpu
-
        /*
         * Compute the delta between the address I am compiled to run at and the
         * address I am actually running at.
@@ -74,6 +71,14 @@ startup_64:
        leaq    _text(%rip), %rbp
        subq    $_text - __START_KERNEL_map, %rbp

+       /* Setup stack for verify_cpu() */
+       movq    stack_start(%rip), %rsp
+       subq    $__START_KERNEL_map, %rsp
+       addq    %rbp, %rsp
+
+       /* Sanitize CPU configuration */
+       call verify_cpu
+
        /* Is the address not 2M aligned? */
        testl   $~PMD_PAGE_MASK, %ebp
        jnz     bad_address