efi/libstub/x86: Use mandatory 16-byte stack alignment in mixed mode

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200113172245.27925-3-ardb@kernel.org

diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
index d040ff5..8fb7f67 100644 (file)
--- 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 */