efi/x86: Don't depend on firmware GDT layout

When booting in mixed mode, the firmware's GDT is still installed at
handover entry in efi32_stub_entry. We save the GDTR for later use in
__efi64_thunk but we are assuming that descriptor 2 (__KERNEL_CS) is a
valid 32-bit code segment descriptor and that descriptor 3
(__KERNEL_DS/__BOOT_DS) is a valid data segment descriptor.

This happens to be true for OVMF (it actually uses descriptor 1 for data
segments, but descriptor 3 is also setup as data), but we shouldn't
depend on this being the case.

Fix this by saving the code and data selectors in addition to the GDTR
in efi32_stub_entry, and restoring them in __efi64_thunk before calling
the firmware. The UEFI specification guarantees that selectors will be
flat, so using the DS selector for all the segment registers should be
enough.

We also need to install our own GDT before initializing segment
registers in startup_32, so move the GDT load up to the beginning of the
function.

[ardb: mention mixed mode in the commit log]
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-3-nivedita@alum.mit.eduSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/boot/compressed/efi_thunk_64.S

@@ -54,11 +54,16 @@ SYM_FUNC_START(__efi64_thunk)
 	 * Switch to gdt with 32-bit segments. This is the firmware GDT
 	 * that was installed when the kernel started executing. This
 	 * pointer was saved at the EFI stub entry point in head_64.S.
+	 *
+	 * Pass the saved DS selector to the 32-bit code, and use far return to
+	 * restore the saved CS selector.
 	 */
 	leaq	efi32_boot_gdt(%rip), %rax
 	lgdt	(%rax)
 
-	pushq	$__KERNEL_CS
+	movzwl	efi32_boot_ds(%rip), %edx
+	movzwq	efi32_boot_cs(%rip), %rax
+	pushq	%rax
 	leaq	efi_enter32(%rip), %rax
 	pushq	%rax
 	lretq
@@ -73,6 +78,10 @@ SYM_FUNC_START(__efi64_thunk)
 	movl	%ebx, %es
 	pop	%rbx
 	movl	%ebx, %ds
+	/* Clear out 32-bit selector from FS and GS */
+	xorl	%ebx, %ebx
+	movl	%ebx, %fs
+	movl	%ebx, %gs
 
 	/*
 	 * Convert 32-bit status code into 64-bit.
@@ -92,10 +101,12 @@ SYM_FUNC_END(__efi64_thunk)
  * The stack should represent the 32-bit calling convention.
  */
 SYM_FUNC_START_LOCAL(efi_enter32)
-	movl	$__KERNEL_DS, %eax
-	movl	%eax, %ds
-	movl	%eax, %es
-	movl	%eax, %ss
+	/* Load firmware selector into data and stack segment registers */
+	movl	%edx, %ds
+	movl	%edx, %es
+	movl	%edx, %fs
+	movl	%edx, %gs
+	movl	%edx, %ss
 
 	/* Reload pgtables */
 	movl	%cr3, %eax
@@ -157,6 +168,14 @@ SYM_DATA_START(efi32_boot_gdt)
 	.quad	0
 SYM_DATA_END(efi32_boot_gdt)
 
+SYM_DATA_START(efi32_boot_cs)
+	.word	0
+SYM_DATA_END(efi32_boot_cs)
+
+SYM_DATA_START(efi32_boot_ds)
+	.word	0
+SYM_DATA_END(efi32_boot_ds)
+
 SYM_DATA_START(efi_gdt64)
 	.word	efi_gdt64_end - efi_gdt64
 	.long	0			/* Filled out by user */

arch/x86/boot/compressed/head_64.S

@@ -54,10 +54,6 @@ SYM_FUNC_START(startup_32)
 	 */
 	cld
 	cli
-	movl	$(__BOOT_DS), %eax
-	movl	%eax, %ds
-	movl	%eax, %es
-	movl	%eax, %ss
 
 /*
  * Calculate the delta between where we were compiled to run
@@ -72,10 +68,20 @@ SYM_FUNC_START(startup_32)
 1:	popl	%ebp
 	subl	$1b, %ebp
 
+	/* Load new GDT with the 64bit segments using 32bit descriptor */
+	addl	%ebp, gdt+2(%ebp)
+	lgdt	gdt(%ebp)
+
+	/* Load segment registers with our descriptors */
+	movl	$__BOOT_DS, %eax
+	movl	%eax, %ds
+	movl	%eax, %es
+	movl	%eax, %fs
+	movl	%eax, %gs
+	movl	%eax, %ss
+
 /* setup a stack and make sure cpu supports long mode. */
-	movl	$boot_stack_end, %eax
-	addl	%ebp, %eax
-	movl	%eax, %esp
+	leal	boot_stack_end(%ebp), %esp
 
 	call	verify_cpu
 	testl	%eax, %eax
@@ -112,10 +118,6 @@ SYM_FUNC_START(startup_32)
  * Prepare for entering 64 bit mode
  */
 
-	/* Load new GDT with the 64bit segments using 32bit descriptor */
-	addl	%ebp, gdt+2(%ebp)
-	lgdt	gdt(%ebp)
-
 	/* Enable PAE mode */
 	movl	%cr4, %eax
 	orl	$X86_CR4_PAE, %eax
@@ -232,9 +234,13 @@ SYM_FUNC_START(efi32_stub_entry)
 
 	movl	%ecx, efi32_boot_args(%ebp)
 	movl	%edx, efi32_boot_args+4(%ebp)
-	sgdtl	efi32_boot_gdt(%ebp)
 	movb	$0, efi_is64(%ebp)
 
+	/* Save firmware GDTR and code/data selectors */
+	sgdtl	efi32_boot_gdt(%ebp)
+	movw	%cs, efi32_boot_cs(%ebp)
+	movw	%ds, efi32_boot_ds(%ebp)
+
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax