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

[Pingfan Liu]

On Wed, Feb 27, 2019 at 3:40 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> + Kees.
>
> @Kees, you might want to go upthread a bit for context.
>
Seems not reply from Kees.
> 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.
>
As Dave said, some un-relocatable code should be loaded to a specified
space. Also the param is used by archs beside x86
> > 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.
>
Kernel has already done this excluding 1gb pages. Do we need to worry
about 200-400 MB for crashkernel? And I think if a user specify the
region, then he/she should be aware of the limit of KASLR (can printk
to warn him/her).

> But I might be way off here because of something else I'm missing ...
>
So how do you think about this now? Just leaving a unstable kernel
parameter, or printk some info when crashkernel=x@y fails.

Thanks,
Pingfan
> --
> Regards/Gruss,
>     Boris.
>
> Good mailing practices for 400: avoid top-posting and trim the reply.