linux-kernel - [PATCH] x86/sev: Don't use cc_platform

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Date:   Tue, 23 Aug 2022 16:55:51 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>
CC:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Asish Kalra <ashish.kalra@....com>,
        "Michael Roth" <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        <stable@...r.kernel.org>, "Sean Christopherson" <seanjc@...gle.com>
Subject: [PATCH] x86/sev: Don't use cc_platform_has() for early SEV-SNP calls

When running identity mapped and depending on the kernel configuration,
it is possible that cc_platform_has() can have compiler generated code
that uses jump tables. This causes a boot failure because the jump table
uses un-mapped kernel virtual addresses, not identity mapped addresses.
This has been seen with CONFIG_RETPOLINE=n.

Similar to sme_encrypt_kernel(), use an open-coded direct check for the
status of SNP rather than trying to eliminate the jump table. This
preserves any code optimization in cc_platform_has() that can be useful
post boot. It also limits the changes to SEV-specific files so that
future compiler features won't necessarily require possible build changes
just because they are not compatible with running identity mapped.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216385
Link: https://lore.kernel.org/all/YqfabnTRxFSM+LoX@google.com/
Cc: <stable@...r.kernel.org> # 5.19.x
Reported-by: Sean Christopherson <seanjc@...gle.com>
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/kernel/sev.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 63dc626627a0..4f84c3f11af5 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -701,7 +701,13 @@ static void __init early_set_pages_state(unsigned long paddr, unsigned int npage
 void __init early_snp_set_memory_private(unsigned long vaddr, unsigned long paddr,
 					 unsigned int npages)
 {
-	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
+	/*
+	 * This can be invoked in early boot while running identity mapped, so
+	 * use an open coded check for SNP instead of using cc_platform_has().
+	 * This eliminates worries about jump tables or checking boot_cpu_data
+	 * in the cc_platform_has() function.
+	 */
+	if (!(sev_status & MSR_AMD64_SEV_SNP_ENABLED))
 		return;
 
 	 /*
@@ -717,7 +723,13 @@ void __init early_snp_set_memory_private(unsigned long vaddr, unsigned long padd
 void __init early_snp_set_memory_shared(unsigned long vaddr, unsigned long paddr,
 					unsigned int npages)
 {
-	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
+	/*
+	 * This can be invoked in early boot while running identity mapped, so
+	 * use an open coded check for SNP instead of using cc_platform_has().
+	 * This eliminates worries about jump tables or checking boot_cpu_data
+	 * in the cc_platform_has() function.
+	 */
+	if (!(sev_status & MSR_AMD64_SEV_SNP_ENABLED))
 		return;
 
 	/* Invalidate the memory pages before they are marked shared in the RMP table. */
-- 
2.37.2