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x86/efi/mixed: Remove dependency on legacy startup_32 code

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The EFI mixed mode startup code calls into startup_32 in the legacy
decompressor with a mocked up boot_params struct, only to get it to set
up the 1:1 mapping of the lower 4 GiB of memory and switch to a GDT that
supports 64-bit mode.

In order to be able to reuse the EFI mixed mode startup code in EFI
zboot images, which do not incorporate the legacy decompressor code,
decouple it, by dealing with the GDT and IDT directly.

Doing so makes it possible to construct a GDT that is compatible with
the one the firmware uses, with one additional entry for a 64-bit mode
code segment appended. This removes the need entirely to switch between
GDTs and IDTs or data segment selector values and all of this code can
be removed.

Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
This commit is contained in:
Ard Biesheuvel
2025-02-21 16:54:31 +01:00
parent d545e182a8
commit 6e2da8d87c
2 changed files with 62 additions and 166 deletions
Download Patch File Download Diff File Expand all files Collapse all files
arch/x86/boot/compressed/efi_mixed.S
+62 -159
View File
@@ -15,70 +15,23 @@
*/
#include <linux/linkage.h>
#include <asm/asm-offsets.h>
#include <asm/desc_defs.h>
#include <asm/msr.h>
#include <asm/page_types.h>
#include <asm/pgtable_types.h>
#include <asm/processor-flags.h>
#include <asm/segment.h>
#include <asm/setup.h>
.code64
.text
/*
* When booting in 64-bit mode on 32-bit EFI firmware, startup_64_mixed_mode()
* is the first thing that runs after switching to long mode. Depending on
* whether the EFI handover protocol or the compat entry point was used to
* enter the kernel, it will either branch to the common 64-bit EFI stub
* entrypoint efi_stub_entry() directly, or via the 64-bit EFI PE/COFF
* entrypoint efi_pe_entry(). In the former case, the bootloader must provide a
* struct bootparams pointer as the third argument, so the presence of such a
* pointer is used to disambiguate.
*
* +--------------+
* +------------------+ +------------+ +------>| efi_pe_entry |
* | efi32_pe_entry |---->| | | +-----------+--+
* +------------------+ | | +------+----------------+ |
* | startup_32 |---->| startup_64_mixed_mode | |
* +------------------+ | | +------+----------------+ |
* | efi32_stub_entry |---->| | | |
* +------------------+ +------------+ | |
* V |
* +------------+ +----------------+ |
* | startup_64 |<----| efi_stub_entry |<--------+
* +------------+ +----------------+
*/
SYM_FUNC_START(startup_64_mixed_mode)
lea efi32_boot_args(%rip), %rdx
mov 0(%rdx), %edi
mov 4(%rdx), %esi
leaq (pte + 5 * PAGE_SIZE)(%rip), %rax
movq %rax, %cr3 // reload after startup_32
/* Switch to the firmware's stack */
movl efi32_boot_sp(%rip), %esp
andl $~7, %esp
mov 8(%rdx), %edx // saved bootparams pointer
call efi_stub_entry
SYM_FUNC_END(startup_64_mixed_mode)
SYM_FUNC_START(__efi64_thunk)
push %rbp
push %rbx
movl %ds, %eax
push %rax
movl %es, %eax
push %rax
movl %ss, %eax
push %rax
/* Copy args passed on stack */
movq 0x30(%rsp), %rbp
movq 0x38(%rsp), %rbx
movq 0x40(%rsp), %rax
movq 0x18(%rsp), %rbp
movq 0x20(%rsp), %rbx
movq 0x28(%rsp), %rax
/*
* Convert x86-64 ABI params to i386 ABI
@@ -93,44 +46,14 @@ SYM_FUNC_START(__efi64_thunk)
movl %ebx, 0x18(%rsp)
movl %eax, 0x1c(%rsp)
leaq 0x20(%rsp), %rbx
sgdt (%rbx)
sidt 16(%rbx)
leaq 1f(%rip), %rbp
movl %cs, %ebx
/*
* Switch to IDT and GDT with 32-bit segments. These are the firmware
* GDT and IDT that were installed when the kernel started executing.
* The pointers were saved by the efi32_entry() routine below.
*
* Pass the saved DS selector to the 32-bit code, and use far return to
* restore the saved CS selector.
*/
lidt efi32_boot_idt(%rip)
lgdt efi32_boot_gdt(%rip)
movzwl efi32_boot_ds(%rip), %edx
movzwq efi32_boot_cs(%rip), %rax
pushq %rax
leaq efi_enter32(%rip), %rax
pushq %rax
lretq
ljmpl *efi32_call(%rip)
1: addq $64, %rsp
movq %rdi, %rax
pop %rbx
movl %ebx, %ss
pop %rbx
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
pop %rbx
pop %rbp
RET
@@ -141,7 +64,6 @@ SYM_FUNC_END(__efi64_thunk)
SYM_FUNC_START(efi32_stub_entry)
call 1f
1: popl %ecx
leal (efi32_boot_args - 1b)(%ecx), %ebx
/* Clear BSS */
xorl %eax, %eax
@@ -153,11 +75,8 @@ SYM_FUNC_START(efi32_stub_entry)
rep stosl
add $0x4, %esp /* Discard return address */
popl %ecx
popl %edx
popl %esi
movl %esi, 8(%ebx)
jmp efi32_entry
movl 8(%esp), %ebx /* struct boot_params pointer */
jmp efi32_startup
SYM_FUNC_END(efi32_stub_entry)
#endif
@@ -167,13 +86,6 @@ SYM_FUNC_END(efi32_stub_entry)
* The stack should represent the 32-bit calling convention.
*/
SYM_FUNC_START_LOCAL(efi_enter32)
/* Load firmware selector into data and stack segment registers */
movl %edx, %ds
movl %edx, %es
movl %edx, %fs
movl %edx, %gs
movl %edx, %ss
/* Disable paging */
movl %cr0, %eax
btrl $X86_CR0_PG_BIT, %eax
@@ -190,21 +102,9 @@ SYM_FUNC_START_LOCAL(efi_enter32)
/* We must preserve return value */
movl %eax, %edi
/*
* Some firmware will return with interrupts enabled. Be sure to
* disable them before we switch GDTs and IDTs.
*/
cli
lidtl 16(%ebx)
lgdtl (%ebx)
xorl %eax, %eax
lldt %ax
call efi32_enable_long_mode
pushl $__KERNEL_CS
pushl %ebx
pushl %ebp
lret
SYM_FUNC_END(efi_enter32)
@@ -230,50 +130,56 @@ SYM_FUNC_START_LOCAL(efi32_enable_long_mode)
SYM_FUNC_END(efi32_enable_long_mode)
/*
* This is the common EFI stub entry point for mixed mode.
* This is the common EFI stub entry point for mixed mode. It sets up the GDT
* and page tables needed for 64-bit execution, after which it calls the
* common 64-bit EFI entrypoint efi_stub_entry().
*
* Arguments: %ecx image handle
* %edx EFI system table pointer
* Arguments: 0(%esp) image handle
* 4(%esp) EFI system table pointer
* %ebx struct boot_params pointer (or NULL)
*
* Since this is the point of no return for ordinary execution, no registers
* are considered live except for the function parameters. [Note that the EFI
* stub may still exit and return to the firmware using the Exit() EFI boot
* service.]
*/
SYM_FUNC_START_LOCAL(efi32_entry)
call 1f
1: pop %ebx
SYM_FUNC_START_LOCAL(efi32_startup)
movl %esp, %ebp
/* Save firmware GDTR and code/data selectors */
sgdtl (efi32_boot_gdt - 1b)(%ebx)
movw %cs, (efi32_boot_cs - 1b)(%ebx)
movw %ds, (efi32_boot_ds - 1b)(%ebx)
subl $8, %esp
sgdtl (%esp) /* Save GDT descriptor to the stack */
movl 2(%esp), %esi /* Existing GDT pointer */
movzwl (%esp), %ecx /* Existing GDT limit */
inc %ecx /* Existing GDT size */
andl $~7, %ecx /* Ensure size is multiple of 8 */
/* Store firmware IDT descriptor */
sidtl (efi32_boot_idt - 1b)(%ebx)
subl %ecx, %esp /* Allocate new GDT */
andl $~15, %esp /* Realign the stack */
movl %esp, %edi /* New GDT address */
leal 7(%ecx), %eax /* New GDT limit */
pushw %cx /* Push 64-bit CS (for LJMP below) */
pushl %edi /* Push new GDT address */
pushw %ax /* Push new GDT limit */
/* Store firmware stack pointer */
movl %esp, (efi32_boot_sp - 1b)(%ebx)
/* Store boot arguments */
leal (efi32_boot_args - 1b)(%ebx), %ebx
movl %ecx, 0(%ebx)
movl %edx, 4(%ebx)
movb $0x0, 12(%ebx) // efi_is64
/*
* Allocate some memory for a temporary struct boot_params, which only
* needs the minimal pieces that startup_32() relies on.
*/
subl $PARAM_SIZE, %esp
movl %esp, %esi
movl $PAGE_SIZE, BP_kernel_alignment(%esi)
movl $_end - 1b, BP_init_size(%esi)
subl $startup_32 - 1b, BP_init_size(%esi)
/* Copy GDT to the stack and add a 64-bit code segment at the end */
movl $GDT_ENTRY(DESC_CODE64, 0, 0xfffff) & 0xffffffff, (%edi,%ecx)
movl $GDT_ENTRY(DESC_CODE64, 0, 0xfffff) >> 32, 4(%edi,%ecx)
shrl $2, %ecx
cld
rep movsl /* Copy the firmware GDT */
lgdtl (%esp) /* Switch to the new GDT */
call 1f
1: pop %edi
/* Record mixed mode entry */
movb $0x0, (efi_is64 - 1b)(%edi)
/* Set up indirect far call to re-enter 32-bit mode */
leal (efi32_call - 1b)(%edi), %eax
addl %eax, (%eax)
movw %cs, 4(%eax)
/* Disable paging */
movl %cr0, %eax
btrl $X86_CR0_PG_BIT, %eax
@@ -297,8 +203,17 @@ SYM_FUNC_START_LOCAL(efi32_entry)
movl %edx, (%eax)
movl %eax, %cr3
jmp startup_32
SYM_FUNC_END(efi32_entry)
call efi32_enable_long_mode
/* Set up far jump to 64-bit mode (CS is already on the stack) */
leal (efi_stub_entry - 1b)(%edi), %eax
movl %eax, 2(%esp)
movl 0(%ebp), %edi
movl 4(%ebp), %esi
movl %ebx, %edx
ljmpl *2(%esp)
SYM_FUNC_END(efi32_startup)
/*
* efi_status_t efi32_pe_entry(efi_handle_t image_handle,
@@ -313,10 +228,8 @@ SYM_FUNC_START(efi32_pe_entry)
btl $29, %edx // check long mode bit
jnc 1f
leal 8(%esp), %esp // preserve stack alignment
movl (%esp), %ecx // image_handle
movl 4(%esp), %edx // sys_table
jmp efi32_entry // pass %ecx, %edx
// no other registers remain live
xor %ebx, %ebx // no struct boot_params pointer
jmp efi32_startup // only ESP and EBX remain live
1: movl $0x80000003, %eax // EFI_UNSUPPORTED
popl %ebx
RET
@@ -332,20 +245,10 @@ SYM_FUNC_END(efi64_stub_entry)
.data
.balign 8
SYM_DATA_START_LOCAL(efi32_boot_gdt)
.word 0
.quad 0
SYM_DATA_END(efi32_boot_gdt)
SYM_DATA_START_LOCAL(efi32_boot_idt)
.word 0
.quad 0
SYM_DATA_END(efi32_boot_idt)
SYM_DATA_LOCAL(efi32_boot_cs, .word 0)
SYM_DATA_LOCAL(efi32_boot_ds, .word 0)
SYM_DATA_LOCAL(efi32_boot_sp, .long 0)
SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0)
SYM_DATA_START_LOCAL(efi32_call)
.long efi_enter32 - .
.word 0x0
SYM_DATA_END(efi32_call)
SYM_DATA(efi_is64, .byte 1)
.bss
arch/x86/boot/compressed/head_64.S
-7
View File
@@ -263,13 +263,6 @@ SYM_FUNC_START(startup_32)
* used to perform that far jump.
*/
leal rva(startup_64)(%ebp), %eax
#ifdef CONFIG_EFI_MIXED
cmpb $1, rva(efi_is64)(%ebp)
je 1f
leal rva(startup_64_mixed_mode)(%ebp), %eax
1:
#endif
pushl $__KERNEL_CS
pushl %eax