Date: Mon, 9 Jan 2017 14:32:29 -0800 From: Thomas Garnier <thgarnie@...gle.com> To: Ingo Molnar <mingo@...nel.org> 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 On Fri, Jan 6, 2017 at 11:35 PM, Ingo Molnar <mingo@...nel.org> wrote: > > * 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 ... > Coming back on that after a bit more testing. The LTR instruction check if the busy bit is already set, if already set then it will just issue a #GP given a bad selector: [ 0.000000] general protection fault: 0040 [#1] SMP ... [ 0.000000] RIP: 0010:native_load_tr_desc+0x9/0x10 ... [ 0.000000] Call Trace: [ 0.000000] cpu_init+0x2d0/0x3c0 [ 0.000000] trap_init+0x2a2/0x312 [ 0.000000] start_kernel+0x1fb/0x43b [ 0.000000] ? set_init_arg+0x55/0x55 [ 0.000000] ? early_idt_handler_array+0x120/0x120 [ 0.000000] x86_64_start_reservations+0x2a/0x2c [ 0.000000] x86_64_start_kernel+0x13d/0x14c [ 0.000000] start_cpu+0x14/0x14 I assume that's in this part of the pseudo-code: if(!IsWithinDescriptorTableLimit(Source.Offset) || Source.Type != TypeGlobal) Exception(GP(SegmentSelector)); SegmentDescriptor = ReadSegmentDescriptor(); if(!IsForAnAvailableTSS(SegmentDescriptor)) Exception(GP(SegmentSelector)); <---- That's where I got the GP TSSSegmentDescriptor.Busy = 1; <------------------------------------------------------------------ That's the pagefault I get otherwise //Locked read-modify-write operation on the entire descriptor when setting busy flag TaskRegister.SegmentSelector = Source; TaskRegister.SegmentDescriptor.TSSSegmentDescriptor; I assume the best option would be to make the remap read-write for the LTR instruction. What do you think? >> > 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 -- Thomas
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.