[PATCH v4 4/5] x86/head/64: Check SEV encryption before switching to kernel page-table

[Joerg Roedel <joro@8bytes.org>]

From: Joerg Roedel <jroedel@suse.de>

When SEV is enabled the kernel requests the C-Bit position again from
the hypervisor to built 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 the sev_verify_cbit() function before writing the CR3.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 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 7eb2a1c87969..3c417734790f 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 efbb3de472df..bc0833713be9 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);
-- 
2.28.0