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

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Message-Id: <20180829141624.13985-3-bhe@redhat.com>
Date:   Wed, 29 Aug 2018 22:16:23 +0800
From:   Baoquan He <bhe@...hat.com>
To:     tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        kirill.shutemov@...ux.intel.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        kexec@...ts.infradead.org, Baoquan He <bhe@...hat.com>
Subject: [PATCH 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

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
triggered.

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@...hat.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 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;
-- 
2.13.6