Date: Sat, 7 Jan 2017 08:35:53 +0100 From: Ingo Molnar <mingo@...nel.org> To: Thomas Garnier <thgarnie@...gle.com> Cc: Andy Lutomirski <luto@...nel.org>, Arjan van de Ven <arjan@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave@...1.net>, Chen Yucong <slaoub@...il.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Anna-Maria Gleixner <anna-maria@...utronix.de>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Michael Ellerman <mpe@...erman.id.au>, Juergen Gross <jgross@...e.com>, Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location * Thomas Garnier <thgarnie@...gle.com> wrote: > > No, and I had the way this worked on 64-bit wrong. LTR requires an > > available TSS and changes it to busy. So here are my thoughts on how > > this should work: > > > > Let's get rid of any connection between this code and KASLR. Every > > time KASLR makes something work differently, a kitten turns all > > Schrödinger on us. This is moving the GDT to the fixmap, plain and > > simple. For now, make it one page per CPU and don't worry about the > > GDT limit. > > I am all for this change but that's more significant. > > Ingo: What do you think about that? I agree with Andy: as I alluded to earlier as well this should be an unconditional change (tested properly, etc.) that robustifies the GDT mapping for everyone. That KASLR kernels improve too is a happy side effect! > > On 32-bit, we're going to have to make the fixmap GDT be read-write because > > making it read-only will break double-fault handling. > > > > On 64-bit, we can use your trick of temporarily mapping the GDT read-write > > every time we load TR, which should happen very rarely. Alternatively, we can > > reload the *GDT* every time we reload TR, which should be comparably slow. > > This is going to regress performance in the extremely rare case where KVM > > exits to a process that uses ioperm() (I think), but I doubt anyone cares. Or > > maybe we could arrange to never reload TR when GDT points at the fixmap by > > having KVM set the host GDT to the direct version and letting KVM's code to > > reload the GDT switch to the fixmap copy. Please check whether the LTR write generates a page fault to a RO PTE even if the busy bit is already set. LTR is pretty slow which suggests that it's microcode, and microcode is usually not sloppy about such things: i.e. LTR would only generate an unconditional write if there's a compatibility dependency on it. But I could easily be wrong ... > > If we need a quirk to keep the fixmap copy read-write, so be it. > > > > None of this should depend on KASLR. IMO it should happen unconditionally. > > I looked back at the fixmap, and I can see a way it could be done > (using NR_CPUS) like the other fixmap ranges. It would limit the > number of cpus to 512 (there is 2M memory left on fixmap on the > default configuration). That's if we never add any other fixmap on > x64. I don't know if it is an acceptable number and if the fixmap > region could be increased. (128 if we do your kvm trick, of course). > > Ingo: What do you think? I think we should scale the fixmap size flexibly with NR_CPUs on 64-bit, and we should limit CPUs on 32-bit to a reasonable value. I.e. let's just do it, if we run into problems it's all solvable AFAICS. Thanks, Ingo
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