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Date: Wed, 2 Aug 2017 11:05:38 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, 
	Arnd Bergmann <arnd@...db.de>, Matthias Kaehlcke <mka@...omium.org>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, 
	Joerg Roedel <joro@...tes.org>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Borislav Petkov <bp@...e.de>, 
	Christian Borntraeger <borntraeger@...ibm.com>, "Rafael J . Wysocki" <rjw@...ysocki.net>, 
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>, 
	Christoph Lameter <cl@...ux.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, 
	Chris Metcalf <cmetcalf@...lanox.com>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Christopher Li <sparse@...isli.org>, 
	Dou Liyang <douly.fnst@...fujitsu.com>, Masahiro Yamada <yamada.masahiro@...ionext.com>, 
	Daniel Borkmann <daniel@...earbox.net>, Markus Trippelsdorf <markus@...ppelsdorf.de>, 
	Peter Foley <pefoley2@...oley.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Tim Chen <tim.c.chen@...ux.intel.com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, 
	Catalin Marinas <catalin.marinas@....com>, Matthew Wilcox <mawilcox@...rosoft.com>, 
	Michal Hocko <mhocko@...e.com>, Rob Landley <rob@...dley.net>, Jiri Kosina <jkosina@...e.cz>, 
	"H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>, Baoquan He <bhe@...hat.com>, 
	Daniel Micay <danielmicay@...il.com>, "the arch/x86 maintainers" <x86@...nel.org>, 
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, xen-devel@...ts.xenproject.org, 
	kvm list <kvm@...r.kernel.org>, linux-pm <linux-pm@...r.kernel.org>, 
	linux-arch <linux-arch@...r.kernel.org>, linux-sparse@...r.kernel.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC 16/22] x86/percpu: Adapt percpu for PIE support

On Wed, Aug 2, 2017 at 9:56 AM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Aug 2, 2017 at 9:42 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> I noticed that not only we have the problem of gs:0x40 not being
>> accessible. The compiler will default to the fs register if
>> mcmodel=kernel is not set.
>>
>> On the next patch set, I am going to add support for
>> -mstack-protector-guard=global so a global variable can be used
>> instead of the segment register. Similar approach than ARM/ARM64.
>
> While this is probably understood, I have to point out that this would
> be a major regression for the stack protection on x86.

I agree, the optimal solution will be using updated gcc/clang.

>
>> Following this patch, I will work with gcc and llvm to add
>> -mstack-protector-reg=<segment register> support similar to PowerPC.
>> This way we can have gs used even without mcmodel=kernel. Once that's
>> an option, I can setup the GDT as described in the previous email
>> (similar to RFG).
>
> It would be much nicer if we could teach gcc about the percpu area
> instead. This would let us solve the global stack protector problem on
> the other architectures:
> http://www.openwall.com/lists/kernel-hardening/2017/06/27/6

Yes, while I am looking at gcc I will take a look at other
architecture to see if I can help there too.

>
> -Kees
>
> --
> Kees Cook
> Pixel Security



-- 
Thomas

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