linux-kernel - Re: [GIT PULL] x86/kaslr for v3.14

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Message-ID: <20140207144914.GA5949@redhat.com>
Date:	Fri, 7 Feb 2014 09:49:15 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Richard Weinberger <richard.weinberger@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Cong Ding <dinggnu@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mathias Krause <minipli@...glemail.com>,
	Michael Davidson <md@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wei Yongjun <yongjun_wei@...ndmicro.com.cn>,
	Dave Young <dyoung@...hat.com>
Subject: Re: [GIT PULL] x86/kaslr for v3.14

On Fri, Jan 31, 2014 at 08:57:03AM -0800, Kees Cook wrote:

[..]
> I have no intention of that. Mentioned earlier in the thread, hiding
> it from root will be pretty ugly/hard/pointless:
> https://lkml.org/lkml/2014/1/27/287
> I would like to just keep the offset out of dmesg.

[ CC Dave Young ]

Hi Kees,

Dave Young is testing kdump with kaslr enabled. He is facing some issues.

One issue he mentioned is that when second kernel boots, it might be
placed in an area which is outside the reserved area for second kernel.

We reserve a certain memory for second kernel. And modify memory map of
second kernel using memmap=exactmap parameter. Looks like kernel placement
is happening before memmap=exactmap takes effect. And that seems to be
the reason that second kernel can be placed outside the reserved memory.

IOW, memmap=exactmap and kaslr don't work together. Is it possible to
first let memmap=exactmap take affect and then kaslr does its job. Or it
is too late by the time memmap=exactmap is parsed.

As a workaround, Dave is currently using "nokaslr" command line parameter
for second kernel. He is still facing issues where makedumpfile segment
faults. He is looking into it further.

I thought I will atleast bring up with issue of memmap=exactmap and kaslr
being incompatible.

Thanks
Vivek
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