[RFC v2-fix-v2 1/1] x86/boot: Avoid #VE during boot for TDX platforms

[Kuppuswamy Sathyanarayanan]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

Avoid operations which will inject #VE during boot process.
They're easy to avoid and it is less complex than handling
the exceptions.

There are a few MSRs and control register bits which the
kernel normally needs to modify during boot.  But, TDX
disallows modification of these registers to help provide
consistent security guarantees ( and avoid generating #VE
when updating them). Fortunately, TDX ensures that these are
all in the correct state before the kernel loads, which means
the kernel has no need to modify them.

The conditions we need to avoid are:

  * Any writes to the EFER MSR
  * Clearing CR0.NE
  * Clearing CR3.MCE

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---

Changes since RFC v2-fix:
 * Fixed commit and comments as per Dave and Dan's suggestions.
 * Merged CR0.NE related change in pa_trampoline_compat() from patch
   titled "x86/boot: Add a trampoline for APs booting in 64-bit mode"
   to this patch. It belongs in this patch.
 * Merged TRAMPOLINE_32BIT_CODE_SIZE related change from patch titled
   "x86/boot: Add a trampoline for APs booting in 64-bit mode" to this
   patch (since it was wrongly merged to that patch during patch split).

 arch/x86/boot/compressed/head_64.S   | 16 ++++++++++++----
 arch/x86/boot/compressed/pgtable.h   |  2 +-
 arch/x86/kernel/head_64.S            | 20 ++++++++++++++++++--
 arch/x86/realmode/rm/trampoline_64.S | 23 +++++++++++++++++++----
 4 files changed, 50 insertions(+), 11 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index e94874f4bbc1..f848569e3fb0 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -616,12 +616,20 @@ SYM_CODE_START(trampoline_32bit_src)
 	movl	$MSR_EFER, %ecx
 	rdmsr
 	btsl	$_EFER_LME, %eax
+	/* Avoid writing EFER if no change was made (for TDX guest) */
+	jc	1f
 	wrmsr
-	popl	%edx
+1:	popl	%edx
 	popl	%ecx
 
 	/* Enable PAE and LA57 (if required) paging modes */
-	movl	$X86_CR4_PAE, %eax
+	movl	%cr4, %eax
+	/*
+	 * Clear all bits except CR4.MCE, which is preserved.
+	 * Clearing CR4.MCE will #VE in TDX guests.
+	 */
+	andl	$X86_CR4_MCE, %eax
+	orl	$X86_CR4_PAE, %eax
 	testl	%edx, %edx
 	jz	1f
 	orl	$X86_CR4_LA57, %eax
@@ -635,8 +643,8 @@ SYM_CODE_START(trampoline_32bit_src)
 	pushl	$__KERNEL_CS
 	pushl	%eax
 
-	/* Enable paging again */
-	movl	$(X86_CR0_PG | X86_CR0_PE), %eax
+	/* Enable paging again. Avoid clearing X86_CR0_NE for TDX */
+	movl	$(X86_CR0_PG | X86_CR0_NE | X86_CR0_PE), %eax
 	movl	%eax, %cr0
 
 	lret
diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
index 6ff7e81b5628..cc9b2529a086 100644
--- a/arch/x86/boot/compressed/pgtable.h
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -6,7 +6,7 @@
 #define TRAMPOLINE_32BIT_PGTABLE_OFFSET	0
 
 #define TRAMPOLINE_32BIT_CODE_OFFSET	PAGE_SIZE
-#define TRAMPOLINE_32BIT_CODE_SIZE	0x70
+#define TRAMPOLINE_32BIT_CODE_SIZE	0x80
 
 #define TRAMPOLINE_32BIT_STACK_END	TRAMPOLINE_32BIT_SIZE
 
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 04bddaaba8e2..6cf8d126b80a 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -141,7 +141,13 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 1:
 
 	/* Enable PAE mode, PGE and LA57 */
-	movl	$(X86_CR4_PAE | X86_CR4_PGE), %ecx
+	movq	%cr4, %rcx
+	/*
+	 * Clear all bits except CR4.MCE, which is preserved.
+	 * Clearing CR4.MCE will #VE in TDX guests.
+	 */
+	andl	$X86_CR4_MCE, %ecx
+	orl	$(X86_CR4_PAE | X86_CR4_PGE), %ecx
 #ifdef CONFIG_X86_5LEVEL
 	testl	$1, __pgtable_l5_enabled(%rip)
 	jz	1f
@@ -229,13 +235,23 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	/* Setup EFER (Extended Feature Enable Register) */
 	movl	$MSR_EFER, %ecx
 	rdmsr
+	/*
+	 * Preserve current value of EFER for comparison and to skip
+	 * EFER writes if no change was made (for TDX guest)
+	 */
+	movl    %eax, %edx
 	btsl	$_EFER_SCE, %eax	/* Enable System Call */
 	btl	$20,%edi		/* No Execute supported? */
 	jnc     1f
 	btsl	$_EFER_NX, %eax
 	btsq	$_PAGE_BIT_NX,early_pmd_flags(%rip)
-1:	wrmsr				/* Make changes effective */
 
+	/* Avoid writing EFER if no change was made (for TDX guest) */
+1:	cmpl	%edx, %eax
+	je	1f
+	xor	%edx, %edx
+	wrmsr				/* Make changes effective */
+1:
 	/* Setup cr0 */
 	movl	$CR0_STATE, %eax
 	/* Make changes effective */
diff --git a/arch/x86/realmode/rm/trampoline_64.S b/arch/x86/realmode/rm/trampoline_64.S
index 957bb21ce105..cf14d0326a48 100644
--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -143,13 +143,27 @@ SYM_CODE_START(startup_32)
 	movl	%eax, %cr3
 
 	# Set up EFER
+	movl	$MSR_EFER, %ecx
+	rdmsr
+	/*
+	 * Skip writing to EFER if the register already has desiered
+	 * value (to avoid #VE for TDX guest).
+	 */
+	cmp	pa_tr_efer, %eax
+	jne	.Lwrite_efer
+	cmp	pa_tr_efer + 4, %edx
+	je	.Ldone_efer
+.Lwrite_efer:
 	movl	pa_tr_efer, %eax
 	movl	pa_tr_efer + 4, %edx
-	movl	$MSR_EFER, %ecx
 	wrmsr
 
-	# Enable paging and in turn activate Long Mode
-	movl	$(X86_CR0_PG | X86_CR0_WP | X86_CR0_PE), %eax
+.Ldone_efer:
+	/*
+	 * Enable paging and in turn activate Long Mode. Avoid clearing
+	 * X86_CR0_NE for TDX.
+	 */
+	movl	$(X86_CR0_PG | X86_CR0_WP | X86_CR0_NE | X86_CR0_PE), %eax
 	movl	%eax, %cr0
 
 	/*
@@ -169,7 +183,8 @@ SYM_CODE_START(pa_trampoline_compat)
 	movl	$rm_stack_end, %esp
 	movw	$__KERNEL_DS, %dx
 
-	movl	$X86_CR0_PE, %eax
+	/* Avoid clearing X86_CR0_NE for TDX */
+	movl	$(X86_CR0_NE | X86_CR0_PE), %eax
 	movl	%eax, %cr0
 	ljmpl   $__KERNEL32_CS, $pa_startup_32
 SYM_CODE_END(pa_trampoline_compat)
-- 
2.25.1