[PATCH 4/6] x86/compressed: move startup32_check_sev_cbit out of startup code

[Ard Biesheuvel]

Move startup32_check_sev_cbit() out of head_64.S and turn it into an
ordinary function using the ordinary calling conventions, rather than
preserving and restoring the registers that are known to be live at the
call site.

Also reorder the call with the EFI mixed mode check, so we are not
omitting the call to startup32_check_sev_cbit() on mixed mode systems
implicitly, even if the set of machines supporting both features is
empty.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S     | 83 ++------------------
 arch/x86/boot/compressed/mem_encrypt.S | 65 +++++++++++++++
 2 files changed, 70 insertions(+), 78 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 1ca2ed52f93c..382ed3d8b26a 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -251,6 +251,11 @@ SYM_FUNC_START(startup_32)
 	movl    $__BOOT_TSS, %eax
 	ltr	%ax
 
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	/* Check if the C-bit position is correct when SEV is active */
+	call	startup32_check_sev_cbit
+#endif
+
 	/*
 	 * Setup for the jump to 64bit mode
 	 *
@@ -266,9 +271,6 @@ SYM_FUNC_START(startup_32)
 	leal	rva(startup_64_mixedmode)(%ebp), %eax
 	jne	1f
 #endif
-	/* Check if the C-bit position is correct when SEV is active */
-	call	startup32_check_sev_cbit
-
 	leal	rva(startup_64)(%ebp), %eax
 1:
 	pushl	$__KERNEL_CS
@@ -703,81 +705,6 @@ SYM_DATA_START(boot_idt)
 	.endr
 SYM_DATA_END_LABEL(boot_idt, SYM_L_GLOBAL, boot_idt_end)
 
-#ifdef CONFIG_AMD_MEM_ENCRYPT
-	__HEAD
-	.code32
-#endif
-
-/*
- * Check for the correct C-bit position when the startup_32 boot-path is used.
- *
- * The check makes use of the fact that all memory is encrypted when paging is
- * disabled. The function creates 64 bits of random data using the RDRAND
- * instruction. RDRAND is mandatory for SEV guests, so always available. If the
- * hypervisor violates that the kernel will crash right here.
- *
- * The 64 bits of random data are stored to a memory location and at the same
- * time kept in the %eax and %ebx registers. Since encryption is always active
- * when paging is off the random data will be stored encrypted in main memory.
- *
- * Then paging is enabled. When the C-bit position is correct all memory is
- * still mapped encrypted and comparing the register values with memory will
- * succeed. An incorrect C-bit position will map all memory unencrypted, so that
- * the compare will use the encrypted random data and fail.
- */
-SYM_FUNC_START(startup32_check_sev_cbit)
-#ifdef CONFIG_AMD_MEM_ENCRYPT
-	pushl	%eax
-	pushl	%ebx
-	pushl	%ecx
-	pushl	%edx
-
-	/* Check for non-zero sev_status */
-	movl	rva(sev_status)(%ebp), %eax
-	testl	%eax, %eax
-	jz	4f
-
-	/*
-	 * Get two 32-bit random values - Don't bail out if RDRAND fails
-	 * because it is better to prevent forward progress if no random value
-	 * can be gathered.
-	 */
-1:	rdrand	%eax
-	jnc	1b
-2:	rdrand	%ebx
-	jnc	2b
-
-	/* Store to memory and keep it in the registers */
-	movl	%eax, rva(sev_check_data)(%ebp)
-	movl	%ebx, rva(sev_check_data+4)(%ebp)
-
-	/* Enable paging to see if encryption is active */
-	movl	%cr0, %edx			 /* Backup %cr0 in %edx */
-	movl	$(X86_CR0_PG | X86_CR0_PE), %ecx /* Enable Paging and Protected mode */
-	movl	%ecx, %cr0
-
-	cmpl	%eax, rva(sev_check_data)(%ebp)
-	jne	3f
-	cmpl	%ebx, rva(sev_check_data+4)(%ebp)
-	jne	3f
-
-	movl	%edx, %cr0	/* Restore previous %cr0 */
-
-	jmp	4f
-
-3:	/* Check failed - hlt the machine */
-	hlt
-	jmp	3b
-
-4:
-	popl	%edx
-	popl	%ecx
-	popl	%ebx
-	popl	%eax
-#endif
-	RET
-SYM_FUNC_END(startup32_check_sev_cbit)
-
 /*
  * Stack and heap for uncompression
  */
diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index 889450d073ea..3cd3db0da49d 100644
--- a/arch/x86/boot/compressed/mem_encrypt.S
+++ b/arch/x86/boot/compressed/mem_encrypt.S
@@ -241,6 +241,71 @@ SYM_FUNC_START(startup32_load_idt)
 	RET
 SYM_FUNC_END(startup32_load_idt)
 
+/*
+ * Check for the correct C-bit position when the startup_32 boot-path is used.
+ *
+ * The check makes use of the fact that all memory is encrypted when paging is
+ * disabled. The function creates 64 bits of random data using the RDRAND
+ * instruction. RDRAND is mandatory for SEV guests, so always available. If the
+ * hypervisor violates that the kernel will crash right here.
+ *
+ * The 64 bits of random data are stored to a memory location and at the same
+ * time kept in the %eax and %ebx registers. Since encryption is always active
+ * when paging is off the random data will be stored encrypted in main memory.
+ *
+ * Then paging is enabled. When the C-bit position is correct all memory is
+ * still mapped encrypted and comparing the register values with memory will
+ * succeed. An incorrect C-bit position will map all memory unencrypted, so that
+ * the compare will use the encrypted random data and fail.
+ */
+SYM_FUNC_START(startup32_check_sev_cbit)
+	push	%ebp
+	push	%ebx
+
+	call	0f
+0:	pop	%ebp
+
+	/* Check for non-zero sev_status */
+	movl	(sev_status - 0b)(%ebp), %eax
+	testl	%eax, %eax
+	jz	3f
+
+	/*
+	 * Get two 32-bit random values - Don't bail out if RDRAND fails
+	 * because it is better to prevent forward progress if no random value
+	 * can be gathered.
+	 */
+1:	rdrand	%eax
+	jnc	1b
+2:	rdrand	%ebx
+	jnc	2b
+
+	/* Store to memory and keep it in the registers */
+	leal	(sev_check_data - 0b)(%ebp), %ebp
+	movl	%eax, 0(%ebp)
+	movl	%ebx, 4(%ebp)
+
+	/* Enable paging to see if encryption is active */
+	movl	%cr0, %edx			 /* Backup %cr0 in %edx */
+	movl	$(X86_CR0_PG | X86_CR0_PE), %ecx /* Enable Paging and Protected mode */
+	movl	%ecx, %cr0
+
+	cmpl	%eax, 0(%ebp)
+	jne	4f
+	cmpl	%ebx, 4(%ebp)
+	jne	4f
+
+	movl	%edx, %cr0	/* Restore previous %cr0 */
+
+3:	pop	%ebx
+	pop	%ebp
+	RET
+
+4:	/* Check failed - hlt the machine */
+	hlt
+	jmp	4b
+SYM_FUNC_END(startup32_check_sev_cbit)
+
 	.data
 	.balign	8
 SYM_DATA(sme_me_mask,		.quad 0)
-- 
2.35.1