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Subject[tip: x86/seves] x86/head/64: Check SEV encryption before switching to kernel page-table
The following commit has been merged into the x86/seves branch of tip:

Commit-ID: c9f09539e16e281f92a27760fdfae71e8af036f6
Gitweb: https://git.kernel.org/tip/c9f09539e16e281f92a27760fdfae71e8af036f6
Author: Joerg Roedel <jroedel@suse.de>
AuthorDate: Wed, 28 Oct 2020 17:46:58 +01:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 29 Oct 2020 18:09:59 +01:00

x86/head/64: Check SEV encryption before switching to kernel page-table

When SEV is enabled, the kernel requests the C-bit position again from
the hypervisor to build its own page-table. Since the hypervisor is an
untrusted source, the C-bit position needs to be verified before the
kernel page-table is used.

Call sev_verify_cbit() before writing the CR3.

[ bp: Massage. ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lkml.kernel.org/r/20201028164659.27002-5-joro@8bytes.org
---
arch/x86/kernel/head_64.S | 16 ++++++++++++++++
arch/x86/mm/mem_encrypt.c | 1 +
2 files changed, 17 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 7eb2a1c..3c41773 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -161,6 +161,21 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)

/* Setup early boot stage 4-/5-level pagetables. */
addq phys_base(%rip), %rax
+
+ /*
+ * For SEV guests: Verify that the C-bit is correct. A malicious
+ * hypervisor could lie about the C-bit position to perform a ROP
+ * attack on the guest by writing to the unencrypted stack and wait for
+ * the next RET instruction.
+ * %rsi carries pointer to realmode data and is callee-clobbered. Save
+ * and restore it.
+ */
+ pushq %rsi
+ movq %rax, %rdi
+ call sev_verify_cbit
+ popq %rsi
+
+ /* Switch to new page-table */
movq %rax, %cr3

/* Ensure I am executing from virtual addresses */
@@ -279,6 +294,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
SYM_CODE_END(secondary_startup_64)

#include "verify_cpu.S"
+#include "sev_verify_cbit.S"

#ifdef CONFIG_HOTPLUG_CPU
/*
diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
index efbb3de..bc08337 100644
--- a/arch/x86/mm/mem_encrypt.c
+++ b/arch/x86/mm/mem_encrypt.c
@@ -39,6 +39,7 @@
*/
u64 sme_me_mask __section(".data") = 0;
u64 sev_status __section(".data") = 0;
+u64 sev_check_data __section(".data") = 0;
EXPORT_SYMBOL(sme_me_mask);
DEFINE_STATIC_KEY_FALSE(sev_enable_key);
EXPORT_SYMBOL_GPL(sev_enable_key);
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