linux-kernel - Re: [PATCH v2] x86/KASLR: Use old ident map page table if physical randomization failed

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Date:   Fri, 28 Apr 2017 08:25:27 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Dave Young <dyoung@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Yinghai Lu <yinghai@...nel.org>, Borislav Petkov <bp@...e.de>,
        Dave Jiang <dave.jiang@...el.com>,
        Thomas Garnier <thgarnie@...gle.com>
Subject: Re: [PATCH v2] x86/KASLR: Use old ident map page table if physical
 randomization failed


* Baoquan He <bhe@...hat.com> wrote:

> Dave found kdump kernel with kaslr enabled will reset to bios immediately
> if physical randomization failed to find a new position for kernel. But
> kernel with 'nokaslr' option works in this case.
> 
> The reason is kaslr will install a new page table for ident mapping,
> while it missed to consider building ident mapping for original area
> of kernel if kaslr failed on physical randomization.
> 
> This only happens in kexec/kdump kernel. Since ident mapping has been
> built for kexec/kdump in 1st kernel for the whole memory by calling
> init_pgtable(). Here if physical randomizaiton failed, it won't build
> ident mapping for the original area of kernel but change to new page
> table '_pgtable'. Then kernel will reset to bios immediately caused by
> no ident mapping.
> 
> While normal kernel won't be impacted because it comes here via
> startup_32() and cr3 will be _pgtable already. In startup_32() ident
> mapping is built for 0~4G area. In kaslr We just append to the existing
> area instead of entirely overwriting it for on-demand ident mapping
> building. So ident mapping for the original area of kernel is still
> there.
> 
> So for fixing it,  We just switch to the new ident mapping page table
> when physical kaslr succeeds. Otherwise we keep the old page table
> unchanged just like nokaslr does.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> Signed-off-by: Dave Young <dyoung@...hat.com>
> Acked-by: Kees Cook <keescook@...omium.org>

That's not a valid signoff chain. I made it:

 Reported-by: Dave Young <dyoung@...hat.com>
 Signed-off-by: Baoquan He <bhe@...hat.com>
 Acked-by: Dave Young <dyoung@...hat.com>
 Acked-by: Kees Cook <keescook@...omium.org>

(Let me know if that's not the proper authorship chain.)

Thanks,

	Ingo