x86/kexec/64: Prevent kexec from 5-level paging to a 4-level only kernel

If the running kernel has 5-level paging activated, the 5-level paging mode
is preserved across kexec. If the kexec'ed kernel does not contain support
for handling active 5-level paging mode in the decompressor, the
decompressor will crash with #GP.

Prevent this situation at load time. If 5-level paging is active, check the
xloadflags whether the kexec kernel can handle 5-level paging at least in
the decompressor. If not, reject the load attempt and print out an error
message.
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: dyoung@redhat.com
Link: https://lkml.kernel.org/r/20190524073810.24298-3-bhe@redhat.com

arch/x86/kernel/kexec-bzimage64.c

@@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
 		return ret;
 	}
 
+	if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
+		pr_err("bzImage cannot handle 5-level paging mode.\n");
+		return ret;
+	}
+
 	/* I've got a bzImage */
 	pr_debug("It's a relocatable bzImage64\n");
 	ret = 0;