Re: [PATCH v4 70/75] x86/head/64: Don't call verify_cpu() on starting APs

Kees Cook
2020-07-15T01:40:46Z

On Tue, Jul 14, 2020 at 02:09:12PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> The APs are not ready to handle exceptions when verify_cpu() is called
> in secondary_startup_64.

Eek, no. MSR_IA32_MISC_ENABLE_XD_DISABLE needs to be cleared very early
during CPU startup; this can't just be skipped.

Also, is UNWIND_HINT_EMPTY needed for the new target?

-Kees

>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/include/asm/realmode.h | 1 +
> arch/x86/kernel/head_64.S | 1 +
> arch/x86/realmode/init.c | 6 ++++++
> 3 files changed, 8 insertions(+)
>
> diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
> index 6590394af309..5c97807c38a4 100644
> --- a/arch/x86/include/asm/realmode.h
> +++ b/arch/x86/include/asm/realmode.h
> @@ -69,6 +69,7 @@ extern unsigned char startup_32_smp[];
> extern unsigned char boot_gdt[];
> #else
> extern unsigned char secondary_startup_64[];
> +extern unsigned char secondary_startup_64_no_verify[];
> #endif
>
> static inline size_t real_mode_size_needed(void)
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 5b577d6bce7a..8b43ed0592e8 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -165,6 +165,7 @@ SYM_CODE_START(secondary_startup_64)
> /* Sanitize CPU configuration */
> call verify_cpu
>
> +SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> /*
> * Retrieve the modifier (SME encryption mask if SME is active) to be
> * added to the initial pgdir entry that will be programmed into CR3.
> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> index 61a52b925d15..df701f87ddef 100644
> --- a/arch/x86/realmode/init.c
> +++ b/arch/x86/realmode/init.c
> @@ -46,6 +46,12 @@ static void sme_sev_setup_real_mode(struct trampoline_header *th)
> th->flags |= TH_FLAGS_SME_ACTIVE;
>
> if (sev_es_active()) {
> + /*
> + * Skip the call to verify_cpu() in secondary_startup_64 as it
> + * will cause #VC exceptions when the AP can't handle them yet.
> + */
> + th->start = (u64) secondary_startup_64_no_verify;
> +
> if (sev_es_setup_ap_jump_table(real_mode_header))
> panic("Failed to update SEV-ES AP Jump Table");
> }
> --
> 2.27.0
>

--
Kees Cook