[tip: x86/seves] x86/head/64: Check SEV encryption before switching to kernel page-table

[tip-bot2 for Joerg Roedel]

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@xxxxxxx>
AuthorDate:    Wed, 28 Oct 2020 17:46:58 +01:00
Committer:     Borislav Petkov <bp@xxxxxxx>
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@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Link: https://lkml.kernel.org/r/20201028164659.27002-5-joro@xxxxxxxxxx
---
 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);