x86/efi: Preserve segment registers in mixed mode

I was triggering a #GP(0) from userland when running with
CONFIG_EFI_MIXED and CONFIG_IA32_EMULATION, from what looked like
register corruption. Turns out that the mixed mode code was trashing the
contents of %ds, %es and %ss in __efi64_thunk().

Save and restore the contents of these segment registers across the call
to __efi64_thunk() so that we don't corrupt the CPU context.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/arch/x86/platform/efi/efi_stub_64.S b/arch/x86/platform/efi/efi_stub_64.S
index 65b787a9fc4e..e0984ef0374b 100644
--- a/arch/x86/platform/efi/efi_stub_64.S
+++ b/arch/x86/platform/efi/efi_stub_64.S
@@ -176,6 +176,13 @@ ENDPROC(efi_call6)
  * This function must be invoked with a 1:1 mapped stack.
  */
 ENTRY(__efi64_thunk)
+        movl    %ds, %eax
+        push    %rax
+        movl    %es, %eax
+        push    %rax
+        movl    %ss, %eax
+        push    %rax
+
         subq    $32, %rsp
         movl    %esi, 0x0(%rsp)
         movl    %edx, 0x4(%rsp)
@@ -191,7 +198,7 @@ ENTRY(__efi64_thunk)
         movq    %rbx, func_rt_ptr(%rip)
 
         /* Switch to gdt with 32-bit segments */
-        movl    40(%rsp), %eax
+        movl    64(%rsp), %eax
         lgdt    (%rax)
 
         leaq    efi_enter32(%rip), %rax
@@ -203,6 +210,13 @@ ENTRY(__efi64_thunk)
 
         lgdt    save_gdt(%rip)
 
+        pop     %rbx
+        movl    %ebx, %ss
+        pop     %rbx
+        movl    %ebx, %es
+        pop     %rbx
+        movl    %ebx, %ds
+
         /*
          * Convert 32-bit status code into 64-bit.
          */
@@ -218,11 +232,6 @@ ENTRY(__efi64_thunk)
 ENDPROC(__efi64_thunk)
 
 ENTRY(efi_exit32)
-        xorq    %rax, %rax
-        movl    %eax, %ds
-        movl    %eax, %es
-        movl    %eax, %ss
-
         movq    func_rt_ptr(%rip), %rax
         push    %rax
         mov     %rdi, %rax
@@ -267,7 +276,7 @@ ENTRY(efi_enter32)
          */
         cli
 
-        movl    44(%esp), %eax
+        movl    68(%esp), %eax
         movl    %eax, 2(%eax)
         lgdtl   (%eax)
 
@@ -286,7 +295,7 @@ ENTRY(efi_enter32)
         xorl    %eax, %eax
         lldt    %ax
 
-        movl    48(%esp), %eax
+        movl    72(%esp), %eax
         pushl   $__KERNEL_CS
         pushl   %eax