x86/boot: Reload GDTR after copying to the end of the buffer

The GDT may get overwritten during the copy or during extract_kernel,
which will cause problems if any segment register is touched before the
GDTR is reloaded by the decompressed kernel. For safety update the GDTR
to point to the GDT within the copied kernel.
Signed-off-by: NArvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-4-nivedita@alum.mit.eduSigned-off-by: NArd Biesheuvel <ardb@kernel.org>

arch/x86/boot/compressed/head_64.S

@@ -439,6 +439,16 @@ trampoline_return:
 	cld
 	popq	%rsi
 
+	/*
+	 * 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.
+	 */
+	leaq	gdt64(%rbx), %rax
+	subq	%rbp, 2(%rax)
+	addq	%rbx, 2(%rax)
+	lgdt	(%rax)
+
 /*
  * Jump to the relocated address.
  */