linux-kernel - Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for kexec kernel

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Date:   Fri, 28 Jul 2023 18:55:35 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tao Liu <ltao@...hat.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, ardb@...nel.org,
        linux-kernel@...r.kernel.org, bhe@...hat.com, dyoung@...hat.com,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for
 kexec kernel

On Thu, Jul 27, 2023 at 07:03:26PM +0800, Tao Liu wrote:
> Hi Borislav,
> 
> Sorry for the late response. I spent some time retesting your patch
> against 6.5.0-rc1 and 6.5.0-rc3, and it is OK. So
> 
> Reported-and-tested-by: Tao Liu <ltao@...hat.com>
> 
> And will we use this patch as a workaround or will we wait for a
> better solution as proposed by Michael?

First of all, please do not top-post.

And yes, here's a better one. I'd appreciate it you testing it.

Thx.

---
 arch/x86/boot/compressed/idt_64.c |  5 ++++-
 arch/x86/boot/compressed/sev.c    | 37 +++++++++++++++++++++++++++++--
 2 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/idt_64.c b/arch/x86/boot/compressed/idt_64.c
index 6debb816e83d..0f03ac12e2a6 100644
--- a/arch/x86/boot/compressed/idt_64.c
+++ b/arch/x86/boot/compressed/idt_64.c
@@ -63,7 +63,10 @@ void load_stage2_idt(void)
 	set_idt_entry(X86_TRAP_PF, boot_page_fault);
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
-	set_idt_entry(X86_TRAP_VC, boot_stage2_vc);
+	if (sev_status & BIT(1))
+		set_idt_entry(X86_TRAP_VC, boot_stage2_vc);
+	else
+		set_idt_entry(X86_TRAP_VC, NULL);
 #endif
 
 	load_boot_idt(&boot_idt_desc);
diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 09dc8c187b3c..c3e343bd4760 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -404,13 +404,46 @@ void sev_enable(struct boot_params *bp)
 	if (bp)
 		bp->cc_blob_address = 0;
 
+	/*
+	 * Do an initial SEV capability check before snp_init() which
+	 * loads the CPUID page and the same checks afterwards are done
+	 * without the hypervisor and are trustworthy.
+	 *
+	 * If the HV fakes SEV support, the guest will crash'n'burn
+	 * which is good enough.
+	 */
+
+	/* Check for the SME/SEV support leaf */
+	eax = 0x80000000;
+	ecx = 0;
+	native_cpuid(&eax, &ebx, &ecx, &edx);
+	if (eax < 0x8000001f)
+		return;
+
+	/*
+	 * Check for the SME/SEV feature:
+	 *   CPUID Fn8000_001F[EAX]
+	 *   - Bit 0 - Secure Memory Encryption support
+	 *   - Bit 1 - Secure Encrypted Virtualization support
+	 *   CPUID Fn8000_001F[EBX]
+	 *   - Bits 5:0 - Pagetable bit position used to indicate encryption
+	 */
+	eax = 0x8000001f;
+	ecx = 0;
+	native_cpuid(&eax, &ebx, &ecx, &edx);
+	/* Check whether SEV is supported */
+	if (!(eax & BIT(1)))
+		return;
+
 	/*
 	 * Setup/preliminary detection of SNP. This will be sanity-checked
 	 * against CPUID/MSR values later.
 	 */
 	snp = snp_init(bp);
 
-	/* Check for the SME/SEV support leaf */
+	/* Now repeat the checks with the SNP CPUID table. */
+
+	/* Recheck the SME/SEV support leaf */
 	eax = 0x80000000;
 	ecx = 0;
 	native_cpuid(&eax, &ebx, &ecx, &edx);
@@ -418,7 +451,7 @@ void sev_enable(struct boot_params *bp)
 		return;
 
 	/*
-	 * Check for the SME/SEV feature:
+	 * Recheck for the SME/SEV feature:
 	 *   CPUID Fn8000_001F[EAX]
 	 *   - Bit 0 - Secure Memory Encryption support
 	 *   - Bit 1 - Secure Encrypted Virtualization support
-- 
2.41.0

-- 
Regards/Gruss,
    Boris.

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