Date: Thu, 5 Jan 2017 13:58:17 -0800 From: Thomas Garnier <thgarnie@...gle.com> To: Andy Lutomirski <luto@...capital.net> 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 Thu, Jan 5, 2017 at 1:19 PM, Andy Lutomirski <luto@...capital.net> wrote: > On Thu, Jan 5, 2017 at 1:08 PM, Thomas Garnier <thgarnie@...gle.com> wrote: >> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@...nel.org> wrote: >>> On Thu, Jan 5, 2017 at 11:03 AM, Thomas Garnier <thgarnie@...gle.com> wrote: >>>> On Thu, Jan 5, 2017 at 10:58 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote: >>>>> On 1/5/2017 9:54 AM, Thomas Garnier wrote: >>>>> >>>>>> >>>>>> That's my goal too. I started by doing a RO remap and got couple >>>>>> problems with hibernation. I can try again for the next iteration or >>>>>> delay it for another patch. I also need to look at KVM GDT usage, I am >>>>>> not familiar with it yet. >>>>> >>>>> >>>>> don't we write to the GDT as part of the TLS segment stuff for glibc ? >>>>> >>>> >>>> Not sure which glibc feature it is. >>>> >>>> In this design, you can write to the GDT per-cpu variable that will >>>> remain read-write. You just need to make the remapping writeable when >>>> we load task registers (ltr) then the processor use the current GDT >>>> address. At least that the case I know, I might find more through >>>> testing. >>> >>> Hmm. I bet that if we preset the accessed bits in all the segments >>> then we don't need it to be writable in general. But your point about >>> set_thread_area (TLS) is well taken. However, I strongly suspect that >>> we could make set_thread_area unconditionally set the accessed bit and >>> no one would ever notice. >> >> Not sure I fully understood and I don't want to miss an important >> point. Do you mean making GDT (remapping and per-cpu) read-only and >> switch the writeable flag only when we write to the per-cpu entry? >> > > What I mean is: write to the GDT through normal percpu access (or > whatever the normal mapping is) but load a read-only alias into the > GDT register. As long as nothing ever tries to write through the GDTR > alias, no page faults will be generated. So we just need to make sure > that nothing ever writes to it through GDTR. AFAIK the only reason > the CPU ever writes to the address in GDTR is to set an accessed bit. > A write is made when we use load_TR_desc (ltr). I didn't see any other yet. > --Andy -- Thomas
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