From: Thomas Garnier Subject: Re: x86: PIE support and option to extend KASLR randomization Date: Tue, 15 Aug 2017 07:20:38 -0700 Message-ID: References: <20170810172615.51965-1-thgarnie@google.com> <20170811124127.kkb5pnkljz4umxuj@gmail.com> <20170815075609.mmzbfwritjzvrpsn@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: Herbert Xu , "David S . Miller" , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Peter Zijlstra , Josh Poimboeuf , Arnd Bergmann , Matthias Kaehlcke , Boris Ostrovsky , Juergen Gross , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Joerg Roedel , Tom Lendacky , Andy Lutomirski , Borislav Petkov , Brian Gerst , "Kirill A . Shutemov" , "Rafael J . Wysocki" , Len Brown , Pavel Machek , Tejun Heo , Christoph La To: Ingo Molnar Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: In-Reply-To: <20170815075609.mmzbfwritjzvrpsn@gmail.com> List-Id: linux-crypto.vger.kernel.org On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar wrote: > > * Thomas Garnier wrote: > >> > Do these changes get us closer to being able to build the kernel as truly >> > position independent, i.e. to place it anywhere in the valid x86-64 address >> > space? Or any other advantages? >> >> Yes, PIE allows us to put the kernel anywhere in memory. It will allow us to >> have a full randomized address space where position and order of sections are >> completely random. There is still some work to get there but being able to build >> a PIE kernel is a significant step. > > So I _really_ dislike the whole PIE approach, because of the huge slowdown: > > +config RANDOMIZE_BASE_LARGE > + bool "Increase the randomization range of the kernel image" > + depends on X86_64 && RANDOMIZE_BASE > + select X86_PIE > + select X86_MODULE_PLTS if MODULES > + default n > + ---help--- > + Build the kernel as a Position Independent Executable (PIE) and > + increase the available randomization range from 1GB to 3GB. > + > + This option impacts performance on kernel CPU intensive workloads up > + to 10% due to PIE generated code. Impact on user-mode processes and > + typical usage would be significantly less (0.50% when you build the > + kernel). > + > + The kernel and modules will generate slightly more assembly (1 to 2% > + increase on the .text sections). The vmlinux binary will be > + significantly smaller due to less relocations. > > To put 10% kernel overhead into perspective: enabling this option wipes out about > 5-10 years worth of painstaking optimizations we've done to keep the kernel fast > ... (!!) Note that 10% is the high-bound of a CPU intensive workload. > > I think the fundamental flaw is the assumption that we need a PIE executable to > have a freely relocatable kernel on 64-bit CPUs. > > Have you considered a kernel with -mcmodel=small (or medium) instead of -fpie > -mcmodel=large? We can pick a random 2GB window in the (non-kernel) canonical > x86-64 address space to randomize the location of kernel text. The location of > modules can be further randomized within that 2GB window. -model=small/medium assume you are on the low 32-bit. It generates instructions where the virtual addresses have the high 32-bit to be zero. I am going to start doing performance testing on -mcmodel=large to see if it is faster than -fPIE. > > It should have far less performance impact than the register-losing and > overhead-inducing -fpie / -mcmodel=large (for modules) execution models. > > My quick guess is tha the performance impact might be close to zero in fact. If mcmodel=small/medium was possible for kernel, I don't think it would have less performance impact than mcmodel=large. It would still need to set the high 32-bit to be a static value, only the relocation would be a different size. > > Thanks, > > Ingo -- Thomas