Date: Thu, 5 Jan 2017 08:39:05 -0800 From: Thomas Garnier <thgarnie@...gle.com> To: Ingo Molnar <mingo@...nel.org> Cc: 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>, Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave@...1.net>, Chen Yucong <slaoub@...il.com>, Arjan van de Ven <arjan@...ux.intel.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>, "the arch/x86 maintainers" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location On Thu, Jan 5, 2017 at 12:11 AM, Ingo Molnar <mingo@...nel.org> wrote: > > * Thomas Garnier <thgarnie@...gle.com> wrote: > >> Each processor holds a GDT in its per-cpu structure. The sgdt >> instruction gives the base address of the current GDT. This address can >> be used to bypass KASLR memory randomization. With another bug, an >> attacker could target other per-cpu structures or deduce the base of the >> main memory section (PAGE_OFFSET). >> >> In this change, a space is reserved at the end of the memory range >> available for KASLR memory randomization. The space is big enough to hold >> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is >> mapped at specific offset based on the target CPU. Note that if there is >> not enough space available, the GDTs are not remapped. >> >> The document was changed to mention GDT remapping for KASLR. This patch >> also include dump page tables support. >> >> This patch was tested on multiple hardware configurations and for >> hibernation support. > >> void kernel_randomize_memory(void); >> +void kernel_randomize_smp(void); >> +void* kaslr_get_gdt_remap(int cpu); > > Yeah, no fundamental objections from me to the principle, but I get some bad vibes > from the naming here: seeing that kernel_randomize_smp() actually makes things > less random. > I agree, I went back and forth on the name. I will change it to something better. > Also, don't we want to do this unconditionally and not allow remapping failures? > > The GDT is fairly small, plus making the SGDT instruction expose fewer kernel > internals would be (marginally) useful on non-randomized kernels as well. > > It also makes the code more common, more predictable, more debuggable and less > complex overall - which is pretty valuable in terms of long term security as well. > Okay, I will add BUG_ON on failures to remap. > Thanks, > > Ingo Ingo: I saw the 5-level page table support being sent through. Do you want me to wait for it to be -next? (Given it will need to be changed too). -- Thomas
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