x86/kexec: Add relocate_kernel() debugging support: Load a GDT

There are some failure modes which lead to triple-faults in the
relocate_kernel() function, which is fairly much undebuggable
for normal mortals.

Adding a GDT in the relocate_kernel() environment is step 1 towards
being able to catch faults and do something more useful.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250312144257.2348250-2-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -40,6 +40,16 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
 
+	.balign 16
+SYM_DATA_START_LOCAL(kexec_debug_gdt)
+	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1
+	.long   0
+	.word   0
+	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
+	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
+	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
+SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
+
 	.section .text..relocate_kernel,"ax";
 	.code64
 SYM_CODE_START_NOALIGN(relocate_kernel)
@@ -116,6 +126,19 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/* store the start address on the stack */
 	pushq   %rdx
 
+	/* Create a GDTR (16 bits limit, 64 bits addr) on stack */
+	leaq	kexec_debug_gdt(%rip), %rax
+	pushq	%rax
+	pushw	(%rax)
+
+	/* Load the GDT, put the stack back */
+	lgdt	(%rsp)
+	addq	$10, %rsp
+
+	/* Test that we can load segments */
+	movq	%ds, %rax
+	movq	%rax, %ds
+
 	/*
 	 * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
 	 * below.