x86/boot/efi: Define __KERNEL32_CS GDT on 64-bit configurations

We would need to switch temporarily to compatibility mode during booting
with 5-level paging enabled. It would require 32-bit code segment
descriptor.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20170606113133.22974-6-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 23 insertions, 2 deletions

diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
index b75c20f779ea..c3e869eaef0c 100644
--- a/arch/x86/boot/compressed/eboot.c
+++ b/arch/x86/boot/compressed/eboot.c
@@ -1046,8 +1046,29 @@ struct boot_params *efi_main(struct efi_config *c,
         memset((char *)gdt->address, 0x0, gdt->size);
         desc = (struct desc_struct *)gdt->address;
 
-        /* The first GDT is a dummy and the second is unused. */
-        desc += 2;
+        /* The first GDT is a dummy. */
+        desc++;
+
+        if (IS_ENABLED(CONFIG_X86_64)) {
+                /* __KERNEL32_CS */
+                desc->limit0 = 0xffff;
+                desc->base0 = 0x0000;
+                desc->base1 = 0x0000;
+                desc->type = SEG_TYPE_CODE | SEG_TYPE_EXEC_READ;
+                desc->s = DESC_TYPE_CODE_DATA;
+                desc->dpl = 0;
+                desc->p = 1;
+                desc->limit = 0xf;
+                desc->avl = 0;
+                desc->l = 0;
+                desc->d = SEG_OP_SIZE_32BIT;
+                desc->g = SEG_GRANULARITY_4KB;
+                desc->base2 = 0x00;
+                desc++;
+        } else {
+                /* Second entry is unused on 32-bit */
+                desc++;
+        }
 
         /* __KERNEL_CS */
         desc->limit0 = 0xffff;