linux-kernel - Re: [PATCH v4 70/75] x86/head/64: Don't call verify

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Message-ID: <202007141837.2B93BBD78@keescook>
Date:   Tue, 14 Jul 2020 18:40:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 70/75] x86/head/64: Don't call verify_cpu() on
 starting APs

On Tue, Jul 14, 2020 at 02:09:12PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.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@...e.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