[PATCH 6.1 122/215] x86/boot/compressed: Move startup32_check_sev_cbit() into .text

Greg Kroah-Hartman
2024-03-04T21:49:17Z

From: Ard Biesheuvel <ardb@kernel.org>

commit b5d854cd4b6a314edd6c15dabc4233b84a0f8e5e upstream.

Move startup32_check_sev_cbit() into the .text section and turn it into
an ordinary function using the ordinary 32-bit calling convention,
instead of saving/restoring the registers that are known to be live at
the only call site. This improves maintainability, and makes it possible
to move this function out of head_64.S and into a separate compilation
unit that is specific to memory encryption.

Note that this requires the call site to be moved before the mixed mode
check, as %eax will be live otherwise.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-14-ardb@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/boot/compressed/head_64.S | 35 +++++++++++++++++++----------------
1 file changed, 19 insertions(+), 16 deletions(-)

--- 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
*
@@ -268,8 +273,6 @@ SYM_FUNC_START(startup_32)
leal rva(startup_64_mixed_mode)(%ebp), %eax
1:
#endif
- /* Check if the C-bit position is correct when SEV is active */
- call startup32_check_sev_cbit

pushl $__KERNEL_CS
pushl %eax
@@ -740,16 +743,17 @@ SYM_DATA_END_LABEL(boot_idt, SYM_L_GLOBA
* succeed. An incorrect C-bit position will map all memory unencrypted, so that
* the compare will use the encrypted random data and fail.
*/
- __HEAD
-SYM_FUNC_START(startup32_check_sev_cbit)
#ifdef CONFIG_AMD_MEM_ENCRYPT
- pushl %eax
+ .text
+SYM_FUNC_START(startup32_check_sev_cbit)
pushl %ebx
- pushl %ecx
- pushl %edx
+ pushl %ebp
+
+ call 0f
+0: popl %ebp

/* Check for non-zero sev_status */
- movl rva(sev_status)(%ebp), %eax
+ movl (sev_status - 0b)(%ebp), %eax
testl %eax, %eax
jz 4f

@@ -764,17 +768,18 @@ SYM_FUNC_START(startup32_check_sev_cbit)
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)
+ 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, rva(sev_check_data)(%ebp)
+ cmpl %eax, 0(%ebp)
jne 3f
- cmpl %ebx, rva(sev_check_data+4)(%ebp)
+ cmpl %ebx, 4(%ebp)
jne 3f

movl %edx, %cr0 /* Restore previous %cr0 */
@@ -786,13 +791,11 @@ SYM_FUNC_START(startup32_check_sev_cbit)
jmp 3b

4:
- popl %edx
- popl %ecx
+ popl %ebp
popl %ebx
- popl %eax
-#endif
RET
SYM_FUNC_END(startup32_check_sev_cbit)
+#endif

/*
* Stack and heap for uncompression