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Subject[PATCH 02/13] efi/libstub/x86: use mandatory 16-byte stack alignment in mixed mode
Date
Reduce the stack frame of the EFI stub's mixed mode thunk routine by
8 bytes, by moving the GDT and return addresses to EBP and EBX, which
we need to preserve anyway, since their top halves will be cleared by
the call into 32-bit firmware code. Doing so results in the UEFI code
being entered with a 16 byte aligned stack, as mandated by the UEFI
spec, fixing the last occurrence in the 64-bit kernel where we violate
this requirement.

Also, move the saved GDT from a global variable to an unused part of the
stack frame, and touch up some other parts of the code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/efi_thunk_64.S | 46 +++++++------------------
1 file changed, 13 insertions(+), 33 deletions(-)

diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
index d040ff5458e5..8fb7f6799c52 100644
--- a/arch/x86/boot/compressed/efi_thunk_64.S
+++ b/arch/x86/boot/compressed/efi_thunk_64.S
@@ -27,12 +27,9 @@ SYM_FUNC_START(__efi64_thunk)
push %rbp
push %rbx

- subq $8, %rsp
- leaq 1f(%rip), %rax
- movl %eax, 4(%rsp)
- leaq efi_gdt64(%rip), %rax
- movl %eax, (%rsp)
- movl %eax, 2(%rax) /* Fixup the gdt base address */
+ leaq 1f(%rip), %rbp
+ leaq efi_gdt64(%rip), %rbx
+ movl %ebx, 2(%rbx) /* Fixup the gdt base address */

movl %ds, %eax
push %rax
@@ -48,12 +45,10 @@ SYM_FUNC_START(__efi64_thunk)
movl %esi, 0x0(%rsp)
movl %edx, 0x4(%rsp)
movl %ecx, 0x8(%rsp)
- movq %r8, %rsi
- movl %esi, 0xc(%rsp)
- movq %r9, %rsi
- movl %esi, 0x10(%rsp)
+ movl %r8d, 0xc(%rsp)
+ movl %r9d, 0x10(%rsp)

- sgdt save_gdt(%rip)
+ sgdt 0x14(%rsp)

/*
* Switch to gdt with 32-bit segments. This is the firmware GDT
@@ -68,11 +63,10 @@ SYM_FUNC_START(__efi64_thunk)
pushq %rax
lretq

-1: addq $32, %rsp
+1: lgdt 0x14(%rsp)
+ addq $32, %rsp
movq %rdi, %rax

- lgdt save_gdt(%rip)
-
pop %rbx
movl %ebx, %ss
pop %rbx
@@ -83,15 +77,9 @@ SYM_FUNC_START(__efi64_thunk)
/*
* Convert 32-bit status code into 64-bit.
*/
- test %rax, %rax
- jz 1f
- movl %eax, %ecx
- andl $0x0fffffff, %ecx
- andl $0xf0000000, %eax
- shl $32, %rax
- or %rcx, %rax
-1:
- addq $8, %rsp
+ roll $1, %eax
+ rorq $1, %rax
+
pop %rbx
pop %rbp
ret
@@ -135,9 +123,7 @@ SYM_FUNC_START_LOCAL(efi_enter32)
*/
cli

- movl 56(%esp), %eax
- movl %eax, 2(%eax)
- lgdtl (%eax)
+ lgdtl (%ebx)

movl %cr4, %eax
btsl $(X86_CR4_PAE_BIT), %eax
@@ -154,9 +140,8 @@ SYM_FUNC_START_LOCAL(efi_enter32)
xorl %eax, %eax
lldt %ax

- movl 60(%esp), %eax
pushl $__KERNEL_CS
- pushl %eax
+ pushl %ebp

/* Enable paging */
movl %cr0, %eax
@@ -172,11 +157,6 @@ SYM_DATA_START(efi32_boot_gdt)
.quad 0
SYM_DATA_END(efi32_boot_gdt)

-SYM_DATA_START_LOCAL(save_gdt)
- .word 0
- .quad 0
-SYM_DATA_END(save_gdt)
-
SYM_DATA_START(efi_gdt64)
.word efi_gdt64_end - efi_gdt64
.long 0 /* Filled out by user */
--
2.20.1
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