Date: Thu, 06 Aug 2020 12:24:58 -0700 From: Kristen Carlson Accardi <kristen@...ux.intel.com> To: Ingo Molnar <mingo@...nel.org> Cc: keescook@...omium.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, arjan@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com Subject: Re: [PATCH v4 00/10] Function Granular KASLR Hi Mingo, thanks for taking a look, I am glad you like the idea. Some replies below: On Thu, 2020-08-06 at 17:32 +0200, Ingo Molnar wrote: > * Kristen Carlson Accardi <kristen@...ux.intel.com> wrote: > > > Function Granular Kernel Address Space Layout Randomization > > (fgkaslr) > > ----------------------------------------------------------------- > > ---- > > > > This patch set is an implementation of finer grained kernel address > > space > > randomization. It rearranges your kernel code at load time > > on a per-function level granularity, with only around a second > > added to > > boot time. > > This is a very nice feature IMO, and it should be far more effective > at randomizing the kernel, due to the sheer number of randomization > bits that kernel function granular randomization presents. > > If this is a good approximation of fg-kaslr randomization depth: > > thule:~/tip> grep ' [tT] ' /proc/kallsyms | wc -l > 88488 > > ... then that's 80K bits of randomization instead of the mere > handful > of kaslr bits we have today. Very nice! > > > In order to hide our new layout, symbols reported through > > /proc/kallsyms will be displayed in a random order. > > Neat. :-) > > > Performance Impact > > ------------------ > > * Run time > > The performance impact at run-time of function reordering varies by > > workload. > > Using kcbench, a kernel compilation benchmark, the performance of a > > kernel > > build with finer grained KASLR was about 1% slower than a kernel > > with standard > > KASLR. Analysis with perf showed a slightly higher percentage of > > L1-icache-load-misses. Other workloads were examined as well, with > > varied > > results. Some workloads performed significantly worse under > > FGKASLR, while > > others stayed the same or were mysteriously better. In general, it > > will > > depend on the code flow whether or not finer grained KASLR will > > impact > > your workload, and how the underlying code was designed. Because > > the layout > > changes per boot, each time a system is rebooted the performance of > > a workload > > may change. > > I'd guess that the biggest performance impact comes from tearing > apart > 'groups' of functions that particular workloads are using. > > In that sense it might be worthwile to add a '__kaslr_group' > function > tag to key functions, which would keep certain performance critical > functions next to each other. > > This shouldn't really be a problem, as even with generous amount of > grouping the number of randomization bits is incredibly large. So my strategy so far was to try to get a very basic non-performance optimized fgkaslr mode merged first, then add performance optimized options as a next step. For example, a user might pass in fgkaslr="group" to the fgkaslr kernel parameter to select a layout which groups some things by whatever criteria we want to mitigate some of the performance impact of full randomization, or they might chose fgkaslr="full", which just randomizes everything (the current implementation). If people think it's worth adding the performance optimizations for the initial merge, I can certainly work on those, but i thought it might be better to keep it super simple at first. > > > Future work could identify hot areas that may not be randomized and > > either > > leave them in the .text section or group them together into a > > single section > > that may be randomized. If grouping things together helps, one > > other thing to > > consider is that if we could identify text blobs that should be > > grouped together > > to benefit a particular code flow, it could be interesting to > > explore > > whether this security feature could be also be used as a > > performance > > feature if you are interested in optimizing your kernel layout for > > a > > particular workload at boot time. Optimizing function layout for a > > particular > > workload has been researched and proven effective - for more > > information > > read the Facebook paper "Optimizing Function Placement for Large- > > Scale > > Data-Center Applications" (see references section below). > > I'm pretty sure the 'grouping' solution would address any real > slowdowns. > > I'd also suggest allowing the passing in of a boot-time pseudo- > random > generator seed number, which would allow the creation of a > pseudo-randomized but repeatable layout across reboots. We talked during the RFC stage of porting the chacha20 code to this early boot stage to use as a prand generator. Ultimately, this means you now have a secret you have to protect (the seed), and so I've dropped this for now. I could see maybe having this as a debug option? I certainly use a prand myself even now when I'm still debugging functional issues (although the one I use for my own debugging isn't suitable for merging). > > > Image Size > > ---------- > > Adding additional section headers as a result of compiling with > > -ffunction-sections will increase the size of the vmlinux ELF file. > > With a standard distro config, the resulting vmlinux was increased > > by > > about 3%. The compressed image is also increased due to the header > > files, > > as well as the extra relocations that must be added. You can expect > > fgkaslr > > to increase the size of the compressed image by about 15%. > > What is the increase of the resulting raw kernel image? Additional > relocations might increase its size (unless I'm missing something) - > it would be nice to measure this effect. I'd expect this to be > really > low. By raw kernel image, do you mean just what eventually gets copied into memory after decompression minus the relocation table? If so, this is almost no difference - the only difference is that there is a little bit of change in the padding between sections vs what the non- randomized kernel is because of alignment differences with the new layout. so you wind up with a few extra bytes give or take. > > vmlinux or compressed kernel size doesn't really matter on x86-64, > it's a boot time only expense well within typical system resource > limits. > > > Disabling > > --------- > > Disabling normal KASLR using the nokaslr command line option also > > disables > > fgkaslr. It is also possible to disable fgkaslr separately by > > booting with > > fgkaslr=off on the commandline. > > I'd suggest to also add a 'nofgkaslr' boot option if it doesn't yet > exist, to keep usage symmetric with kaslr. > > Likewise, there should probably be a 'kaslr=off' option as well. > > The less random our user interfaces are, the better ... > > > arch/x86/boot/compressed/Makefile | 9 +- > > arch/x86/boot/compressed/fgkaslr.c | 811 > > ++++++++++++++++++ > > arch/x86/boot/compressed/kaslr.c | 4 - > > arch/x86/boot/compressed/misc.c | 157 +++- > > arch/x86/boot/compressed/misc.h | 30 + > > arch/x86/boot/compressed/utils.c | 11 + > > arch/x86/boot/compressed/vmlinux.symbols | 17 + > > arch/x86/include/asm/boot.h | 15 +- > > arch/x86/kernel/vmlinux.lds.S | 17 +- > > arch/x86/lib/kaslr.c | 18 +- > > arch/x86/tools/relocs.c | 143 ++- > > arch/x86/tools/relocs.h | 4 +- > > arch/x86/tools/relocs_common.c | 15 +- > > include/asm-generic/vmlinux.lds.h | 18 +- > > include/linux/decompress/mm.h | 12 +- > > include/uapi/linux/elf.h | 1 + > > init/Kconfig | 26 + > > kernel/kallsyms.c | 163 +++- > > kernel/module.c | 81 ++ > > tools/objtool/elf.c | 8 +- > > 26 files changed, 1670 insertions(+), 85 deletions(-) > > create mode 100644 Documentation/security/fgkaslr.rst > > create mode 100644 arch/x86/boot/compressed/fgkaslr.c > > create mode 100644 arch/x86/boot/compressed/utils.c > > create mode 100644 arch/x86/boot/compressed/vmlinux.symbols > > This looks surprisingly lean overall. Most of the changes outside of fgkaslr.c, module.c, and kallsyms.c were little tweaks here and there to accommodate using -ffunction-sections and handling >64K elf sections, otherwise yes, I tried to keep it very self contained and non-invasive.
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