[PATCH v4 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 May 08 2019 - 21:38:13 EST

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

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
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..858cc892672f 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("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;