linux-kernel - Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <878u7ou2i4.fsf@x220.int.ebiederm.org>
Date:	Tue, 29 Sep 2015 20:20:19 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Brian Gerst <brgerst@...il.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Joerg Roedel <joro@...tes.org>, X86 ML <x86@...nel.org>,
	kvm list <kvm@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> Does anyone know what happens if you stick a non-accessed segment in
>> the GDT, map the GDT RO, and access it?
>
> You should get a #PF, as you guess, but go ahead and test it if you
> want to make sure.

I tested this by accident once when workinng on what has become known
as coreboot.  Early in boot with your GDT in a EEPROM switching from
real mode to 32bit protected mode causes a write and locks up the
machine when the hardware declines the write to the GDT to set the
accessed bit.  As I recall the write kept being retried and retried and
retried...

Setting the access bit in the GDT cleared up the problem and I did not
look back.

Way up in 64bit mode something might be different, but I don't know why
cpu designeres would waste the silicon.

Eric
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/