[PATCH 1/1] efi/x86: Use firmware stack for mixed-mode EFI stub

[Arvind Sankar <nivedita@alum.mit.edu>]

The UEFI specification requires a 128KiB stack during boot services. On
a native mode boot, the EFI stub executes on the firmware stack.
However, on a mixed-mode boot, startup_32 switches to the kernel's boot
stack, which is only 16KiB, and the EFI stub is executed with this
stack.

To avoid any potential problems with running out of stack space, save
and restore the UEFI stack pointer in the mixed-mode entry, so that the
EFI stub can use the firmware stack in this case as well.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 arch/x86/boot/compressed/head_64.S | 46 ++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 15 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 4b7ad1dfbea6..923e5c381804 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -237,21 +237,7 @@ SYM_FUNC_START(startup_32)
 	 * used to perform that far jump.
 	 */
 	pushl	$__KERNEL_CS
-	leal	rva(startup_64)(%ebp), %eax
-#ifdef CONFIG_EFI_MIXED
-	movl	rva(efi32_boot_args)(%ebp), %edi
-	cmp	$0, %edi
-	jz	1f
-	leal	rva(efi64_stub_entry)(%ebp), %eax
-	movl	rva(efi32_boot_args+4)(%ebp), %esi
-	movl	rva(efi32_boot_args+8)(%ebp), %edx	// saved bootparams pointer
-	cmpl	$0, %edx
-	jnz	1f
-	leal	rva(efi_pe_entry)(%ebp), %eax
-	movl	%edi, %ecx			// MS calling convention
-	movl	%esi, %edx
-1:
-#endif
+	leal	rva(.L64bit)(%ebp), %eax
 	pushl	%eax
 
 	/* Enter paged protected Mode, activating Long Mode */
@@ -260,6 +246,31 @@ SYM_FUNC_START(startup_32)
 
 	/* Jump from 32bit compatibility mode into 64bit mode. */
 	lret
+
+	.code64
+SYM_INNER_LABEL(.L64bit, SYM_L_LOCAL)
+#ifndef CONFIG_EFI_MIXED
+	jmp	startup_64
+#else
+	movl	efi32_boot_args(%rip), %edi
+	testl	%edi, %edi
+	jz	startup_64
+
+	/* Restore firmware stack pointer */
+	movl	efi32_boot_sp(%rip), %esp
+	/* and align it to 8 mod 16 */
+	andl	$~0xf, %esp
+	subl	$8, %esp
+
+	movl	efi32_boot_args+4(%rip), %esi
+	movl	efi32_boot_args+8(%rip), %edx	// saved bootparams pointer
+	testl	%edx, %edx
+	jnz	efi64_stub_entry
+	movl	%edi, %ecx			// MS calling convention
+	movl	%esi, %edx
+	jmp	efi_pe_entry
+#endif
+	.code32
 SYM_FUNC_END(startup_32)
 
 #ifdef CONFIG_EFI_MIXED
@@ -285,6 +296,9 @@ SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
 	movw	%cs, rva(efi32_boot_cs)(%ebp)
 	movw	%ds, rva(efi32_boot_ds)(%ebp)
 
+	/* Save firmware stack pointer */
+	movl	%esp, rva(efi32_boot_sp)(%ebp)
+
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
@@ -648,6 +662,7 @@ SYM_DATA(image_offset, .long 0)
 
 #ifdef CONFIG_EFI_MIXED
 SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0)
+SYM_DATA_LOCAL(efi32_boot_sp, .long 0)
 SYM_DATA(efi_is64, .byte 1)
 
 #define ST32_boottime		60 // offsetof(efi_system_table_32_t, boottime)
@@ -710,6 +725,7 @@ SYM_FUNC_START(efi32_pe_entry)
 	movl	12(%ebp), %edx			// sys_table
 	movl	-4(%ebp), %esi			// loaded_image
 	movl	LI32_image_base(%esi), %esi	// loaded_image->image_base
+	leave					// clear stack frame
 	movl	%ebx, %ebp			// startup_32 for efi32_pe_stub_entry
 	/*
 	 * We need to set the image_offset variable here since startup_32() will
-- 
2.26.2