linux-kernel - Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

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Message-ID: <542B1EC1.8090502@zytor.com>
Date:	Tue, 30 Sep 2014 14:21:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Baoquan He <bhe@...hat.com>
CC:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, vgoyal@...hat.com, keescook@...omium.org,
	ak@...ux.intel.com, ebiederm@...ssion.com,
	kexec@...ts.infradead.org, whissi@...ssi.de,
	kumagai-atsushi@....nes.nec.co.jp, stable@...r.kernel.org
Subject: Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

On 09/30/2014 12:08 AM, Baoquan He wrote:
> Function handle_relocations() is used to do the relocations handling
> for i686 and kaslr of x86_64. For 32 bit the relocation handling is
> mandotary to perform. For x86_64 only when kaslr is enabled and a
> random kernel location is chosen successfully the relocation handling
> shound be done. However previous implementation only compared the
> kernel loading address and LOAD_PHYSICAL_ADDR where kernel were
> compiled to run at. This would casue system to be exceptional in
> few conditions like when delta between load address and compiled
> address is bigger than what 32bit signed relocations can handle.
> Also there will be limitations that delta can't be too big otherwise
> kernel text virtual addresses will overflow in module address space.
> 
> So in this patch check if kernel location is changed after
> choose_kernel_location() when x86_64. If and only if in x86_64
> and kernel location is changed, we say a kaslr random kernel
> location is chosen, then the relocation handling is needed.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> Acked-by: Vivek Goyal <vgoyal@...hat.com>
> Acked-by: Kees Cook <keescook@...omium.org>
> Tested-by: Thomas D. <whissi@...ssi.de>
> Cc: stable@...r.kernel.org

Could you clarify under what conditions we may end up with 32-bit signed
overflow, and yet have a functional kernel?

	-hpa


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