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

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Message-ID: <20190227073958.GA1786@zn.tnic>
Date:   Wed, 27 Feb 2019 08:39:58 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Baoquan He <bhe@...hat.com>, Kees Cook <keescook@...omium.org>
Cc:     Pingfan Liu <kernelfans@...il.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.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>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86/boot/KASLR: skip the specified crashkernel reserved
 region

+ Kees.

@Kees, you might want to go upthread a bit for context.

On Wed, Feb 27, 2019 at 09:30:34AM +0800, Baoquan He wrote:
> Agree that 'crashkernel=x' should be encouraged to use as the first
> choice when reserve crashkernel. If we decide to not obsolete
> 'crashkernel=x@y', it will leave a unstable kernel parameter.

Is anyone even talking about obsoleting this?

And if anyone is, anyone can think a bit why we can't do this.

> Another worry is that KASLR won't always fail 'crashkernel=x@y',
> customer may set and check in testing stage, then later in production
> environment one time of neglect to not check may cause carashed kernel
> uncaptured.
>
> IMHO, 'crashkernel=x@y' is similar to those specified memmap=ss[#$!]nn
> which have been avoided in boot stage KASLR.

So my worry is that by specifying too many exclusion ranges, we might
limit the kaslr space too much and make it too predictable. Especially
since distros slap those things automatically and most users take them
for granted.

But I might be way off here because of something else I'm missing ...

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.