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Date:   Wed, 22 May 2019 11:20:29 +0800
From:   Dave Young <dyoung@redhat.com>
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org,
        bp@alien8.de, hpa@zytor.com, kirill.shutemov@linux.intel.com,
        x86@kernel.org
Subject: Re: [PATCH v4 2/3] x86/kexec/64: Error out if try to jump to old
 4-level kernel from 5-level kernel
Message-ID: <20190522032029.GB31269@dhcp-128-65.nay.redhat.com>
References: <20190509013644.1246-1-bhe@redhat.com>
 <20190509013644.1246-3-bhe@redhat.com>
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On 05/09/19 at 09:36am, Baoquan He wrote:
> 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>
> ---
>  arch/x86/kernel/kexec-bzimage64.c | 5 +++++

How about the userspace kexec-tools?  It needs a similar detection, but
I'm not sure how to detect paging mode, maybe some sysfs entry or
vmcoreinfo in /proc/vmcore


>  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");

4-level kernel sounds not very clear, maybe something like below?

"5-level paging enabled, can not kexec into an old kernel without 5-level
paging facility"?

> +		return ret;
> +	}
> +
>  	/* I've got a bzImage */
>  	pr_debug("It's a relocatable bzImage64\n");
>  	ret = 0;
> -- 
> 2.17.2
> 

Thanks
Dave