linux-kernel - Re: [PATCH] x86/boot/KASLR: skip the specified crashkernel reserved region

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Message-ID: <20190226104653.GB14836@zn.tnic>
Date:   Tue, 26 Feb 2019 11:46:53 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Baoquan He <bhe@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Nicolas Pitre <nico@...aro.org>,
        Chao Fan <fanc.fnst@...fujitsu.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/boot/KASLR: skip the specified crashkernel reserved
 region

On Tue, Feb 26, 2019 at 11:08:42AM +0800, Pingfan Liu wrote:
> I follow Baoquan's opinion. Due to the randomness caused by KASLR, a
> user may be surprised to find crashkernel=x@y not working sometime.

And she/he will get told in dmesg that the allocation failed.

> If kernel can help them out of this corner automatically, then no
> need to bother them with the message to use alternative method
> crashkernel=M. Anyway it is a cheap method already used by other
> options like hugepages and memmap in handle_mem_options().
> If commitment, then do it without failure. Or just removing
> crashkernel=x@y option on x86.

I can't parse what you're trying to say here but let me repeat myself:
specifying a crashkernel region should not have an influence on KASLR
because this way you limit the kernel where it selects the offset. It
should be other other way around: KASLR offset should be selected and
*then* crashkernel region.

IOW, KASLR offset selection needs to have higher priority than
crashkernel region selection.

-- 
Regards/Gruss,
    Boris.

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