From mboxrd@z Thu Jan 1 00:00:00 1970 From: Denys Vlasenko Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled Date: Mon, 28 Sep 2015 14:45:11 +0200 Message-ID: <56093657.8070409@redhat.com> References: <1443290440-14930-1-git-send-email-dvlasenk@redhat.com> <706E9982-24E3-442B-808A-172909449DD4@zytor.com> <56070241.2030407@redhat.com> <20150928075851.GA23998@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "H. Peter Anvin" , Konrad Rzeszutek Wilk , Boris Ostrovsky , David Vrabel , Joerg Roedel , Gleb Natapov , Paolo Bonzini , kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski , Brian Gerst , Thomas Gleixner , Borislav Petkov , Peter Zijlstra , Linus Torvalds , Andrew Morton , Kees Cook To: Ingo Molnar Return-path: Received: from mx1.redhat.com ([209.132.183.28]:55638 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756780AbbI1MpS (ORCPT ); Mon, 28 Sep 2015 08:45:18 -0400 In-Reply-To: <20150928075851.GA23998@gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On 09/28/2015 09:58 AM, Ingo Molnar wrote: > > * Denys Vlasenko wrote: > >> On 09/26/2015 09:50 PM, H. Peter Anvin wrote: >>> NAK. We really should map the GDT read-only on all 64 bit systems, >>> since we can't hide the address from SLDT. Same with the IDT. >> >> Sorry, I don't understand your point. > > So the problem is that right now the SGDT instruction (which is unprivileged) > leaks the real address of the kernel image: > > fomalhaut:~> ./sgdt > SGDT: ffff88303fd89000 / 007f > > that 'ffff88303fd89000' is a kernel address. Thank you. I do know that SGDT and friends are unprivileged on x86 and thus they allow userspace (and guest kernels in paravirt) learn things they don't need to know. I don't see how making GDT page-aligned and page-sized changes anything in this regard. SGDT will still work, and still leak GDT address. > Your observation in the changelog and your patch: > >>>> It is page-sized because of paravirt. [...] > > ... conflicts with the intention to mark (remap) the primary GDT address read-only > on native kernels as well. > > So what we should do instead is to use the page alignment properly and remap the > GDT to a read-only location, and load that one. If we'd have a small GDT (i.e. what my patch does), we still can remap the entire page which contains small GDT, and simply don't care that some other data is also visible through that RO page. > This would have a couple of advantages: > > - This would give kernel address randomization more teeth on x86. > > - An additional advantage would be that rootkits overwriting the GDT would have > a bit more work to do. > > - A third advantage would be that for NUMA systems we could 'mirror' the GDT into > node-local memory and load those. This makes GDT load cache-misses a bit less > expensive. GDT is per-cpu. Isn't per-cpu memory already NUMA-local?