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 14:59:02 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On 04/28/17 at 08:25am, Ingo Molnar wrote:
> 
> * 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.)

Yeah, this is fine, thanks. Next time I will notice this.