x86/kexec: Restore GDT on return from ::preserve_context kexec

The restore_processor_state() function explicitly states that "the asm code
that gets us here will have restored a usable GDT". That wasn't true in the
case of returning from a ::preserve_context kexec. Make it so.

Without this, the kernel was depending on the called function to reload a
GDT which is appropriate for the kernel before returning.

Test program:

 #include <unistd.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <linux/kexec.h>
 #include <linux/reboot.h>
 #include <sys/reboot.h>
 #include <sys/syscall.h>

 int main (void)
 {
        struct kexec_segment segment = {};
	unsigned char purgatory[] = {
		0x66, 0xba, 0xf8, 0x03,	// mov $0x3f8, %dx
		0xb0, 0x42,		// mov $0x42, %al
		0xee,			// outb %al, (%dx)
		0xc3,			// ret
	};
	int ret;

	segment.buf = &purgatory;
	segment.bufsz = sizeof(purgatory);
	segment.mem = (void *)0x400000;
	segment.memsz = 0x1000;
	ret = syscall(__NR_kexec_load, 0x400000, 1, &segment, KEXEC_PRESERVE_CONTEXT);
	if (ret) {
		perror("kexec_load");
		exit(1);
	}

	ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_KEXEC);
	if (ret) {
		perror("kexec reboot");
		exit(1);
	}
	printf("Success\n");
	return 0;
 }

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241205153343.3275139-2-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -242,6 +242,13 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
 	movq	CR0(%r8), %r8
 	movq	%rax, %cr3
 	movq	%r8, %cr0
+
+#ifdef CONFIG_KEXEC_JUMP
+	/* Saved in save_processor_state. */
+	movq    $saved_context, %rax
+	lgdt    saved_context_gdt_desc(%rax)
+#endif
+
 	movq	%rbp, %rax
 
 	popf