x86/boot/compressed: Fix reloading of GDTR post-relocation

The following commit:

  ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")

introduced GDT setup into the 32-bit kernel's startup_32, and reloads
the GDTR after relocating the kernel for paranoia's sake.

A followup commit:

   32d009137a56 ("x86/boot: Reload GDTR after copying to the end of the buffer")

introduced a similar GDTR reload in the 64-bit kernel as well.

The GDTR is adjusted by (init_size-_end), however this may not be the
correct offset to apply if the kernel was loaded at a misaligned address
or below LOAD_PHYSICAL_ADDR, as in that case the decompression buffer
has an additional offset from the original load address.

This should never happen for a conformant bootloader, but we're being
paranoid anyway, so just store the new GDT address in there instead of
adding any offsets, which is simpler as well.

Fixes: ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
Fixes: 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of the buffer")
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Link: https://lore.kernel.org/r/20200226230031.3011645-2-nivedita@alum.mit.edu

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 356060c..2f8138b 100644 (file)
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -139,12 +139,11 @@ SYM_FUNC_START(startup_32)
        /*
         * The GDT may get overwritten either during the copy we just did or
         * during extract_kernel below. To avoid any issues, repoint the GDTR
-        * to the new copy of the GDT. EAX still contains the previously
-        * calculated relocation offset of init_size - _end.
+        * to the new copy of the GDT.
         */
-       leal    gdt(%ebx), %edx
-       addl    %eax, 2(%edx)
-       lgdt    (%edx)
+       leal    gdt(%ebx), %eax
+       movl    %eax, 2(%eax)
+       lgdt    (%eax)
 
 /*
  * Jump to the relocated address.

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f7bacc4..fcf8fea 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -456,8 +456,8 @@ trampoline_return:
         * to the new copy of the GDT.
         */
        leaq    gdt64(%rbx), %rax
-       subq    %rbp, 2(%rax)
-       addq    %rbx, 2(%rax)
+       leaq    gdt(%rbx), %rdx
+       movq    %rdx, 2(%rax)
        lgdt    (%rax)
 
 /*