[PATCH v5 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

[Baoquan He <bhe@redhat.com>]

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 error
message.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
v4->v5:
  Update the output error message per tglx's comment.

 arch/x86/kernel/kexec-bzimage64.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index 22f60dd26460..7f439739ea3d 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/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;
-- 
2.17.2