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

[tip-bot for Baoquan He <tipbot@zytor.com>]

Commit-ID:  ee338b9ee2822e65a85750da6129946c14962410
Gitweb:     https://git.kernel.org/tip/ee338b9ee2822e65a85750da6129946c14962410
Author:     Baoquan He <bhe@redhat.com>
AuthorDate: Fri, 24 May 2019 15:38:09 +0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 28 Jun 2019 07:14:59 +0200

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 | 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;