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

From: Baoquan He
Date: Wed Aug 29 2018 - 10:16:43 EST

In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
it will check if the booting kernel is in 4-level or 5-level paging
mode, and handle accordingly. However, the old kernel which doesn't
contain the 5-level codes doesn't know how to cope with it, then #GP

Instead of triggering #GP during kexec kernel boot, error out during
kexec loading if find out we are trying to jump to old 4-level kernel
from 5-level kernel.

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
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 7326078eaa7a..f5fe94ee209a 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -316,6 +316,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
return ret;

+ if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
+ pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");
+ return ret;
+ }
/* I've got a bzImage */
pr_debug("It's a relocatable bzImage64\n");
ret = 0;