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

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Message-ID: <20190226051117.GA5085@dhcp-128-65.nay.redhat.com>
Date:   Tue, 26 Feb 2019 13:11:17 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>, 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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/boot/KASLR: skip the specified crashkernel reserved
 region

On 02/25/19 at 09:05pm, Baoquan He wrote:
> On 02/25/19 at 10:45am, Borislav Petkov wrote:
> > On Mon, Feb 25, 2019 at 03:59:56PM +0800, Pingfan Liu wrote:
> > > crashkernel=x@y option may fail to reserve the required memory region if
> > > KASLR puts kernel into the region. To avoid this uncertainty, making KASLR
> > > skip the required region.
> > 
> > Lemme see if I understand this correctly: supplying crashkernel=X@Y
> > influences where KASLR would put the randomized kernel. And it should be
> > the other way around, IMHO. crashkernel= will have to "work" with KASLR
> > to find a suitable range and if the reservation at Y fails, then we tell
> > the user to try the more relaxed variant crashkernel=M.
> 
> Hmm, asking user to try crashkernel=M is an option. Users may want to
> specify a region for crashkernel reservation, Just I forget in what case
> they want crashkernel=x@y set. In crashkernel=x@y specified case, we may
> truly need to avoid the already specified region.
> 
> Not sure if Dave still remember it. If no need, removing it is also
> good.

I do not know the exact use cases, but long time ago the kernel is not
relocatable this might be a reason.  Even now, there could be some non
linux use cases, if the loaded binary is not relocatable then the param
is still useful.

Also this is a general param instead of x86 only, some other arches
still use it, and no crashkernel=X implemented.

Thanks
Dave