Hello! This is an RFC for a proof of concept I've been working on to improve KASLR by making it finer grained. I hope you will take a look at this and give me some feedback on the general concept as well as the early implementation. At this point in time, the code is functional, although there is lots of room for optimization and improvement. This patchset applies to 5.5.0-rc7 The TL;DR summary is: This patch set rearranges your kernel code at load time on a per-function level granularity, with only around a second added to boot time. Background ---------- KASLR was merged into the kernel with the objective of increasing the difficulty of code reuse attacks. Code reuse attacks reused existing code snippets to get around existing memory protections. They exploit software bugs which expose addresses of useful code snippets to control the flow of execution for their own nefarious purposes. KASLR moves the entire kernel code text as a unit at boot time in order to make addresses less predictable. The order of the code within the segment is unchanged - only the base address is shifted. There are a few shortcomings to this algorithm. 1. Low Entropy - there are only so many locations the kernel can fit in. This means an attacker could guess without too much trouble. 2. Knowledge of a single address can reveal the offset of the base address, exposing all other locations for a published/known kernel image. 3. Info leaks abound. Finer grained ASLR has been proposed as a way to make ASLR more resistant to info leaks. It is not a new concept at all, and there are many variations possible. Proposed Improvement -------------------- This patch set proposes adding function reordering on top of the existing KASLR base address randomization. The over-arching objective is incremental improvement over what we already have, as well as something that can be merged and deployed with as little disruption to our existing kernel/ecosystem as possible. It is designed to work with the existing solution, although it can be used independently (not sure why you would do that though...). The implementation is really pretty simple, and there are 2 main area where changes occur: * Build time GCC has an option to place functions into individual .text sections. We can use this option to implement function reordering at load time. The final compiled vmlinux retains all the section headers, which can be used to help us find the address ranges of each function. Using this information and an expanded table of relocation addresses, we can shuffle the individual text sections immediately after decompression. You are probably asking yourself how this could possibly work given the number of tables of addresses that exist inside the kernel today for features such as exception handling and kprobes. Most of these tables generate relocations and require a simple update, and some tables that have assumptions about order require sorting after the update. In order to modify these tables, we preserve a few key symbols from the objcopy symbol stripping process for use after shuffling the text segments. Some highlights from the build time changes to look for: The top level kernel Makefile was modified to add the gcc flag if it is supported. For this RFC, I am applying this flag to everything it is possible to randomize. Future work could turn off this flags for selected files or even entire subsystems, although obviously at the cost of security. The relocs tool is updated to add relative relocations. This information previously wasn't included because it wasn't necessary when moving the entire .text segment as a unit. A new file was created to contain a list of symbols that objcopy should keep. We use those symbols at load time as described below. * Load time The boot kernel was modified to parse the vmlinux elf file after decompression to check for our interesting symbols that we kept, and to look for any .text.* sections to randomize. We then shuffle the sections and update any tables that need to be updated or resorted. The existing code which updated relocation addresses was modified to account for not just a fixed delta from the load address, but the offset that the function section was moved to. This requires inspection of each address to see if it was impacted by a randomization. We use a bsearch to make this less horrible on performce. In this patch we utilize a pseudo-random number generator in order to allow for known seeds to be used to create repeatable images across multiple boots. This is purely for debugging - obviously not recommended for use in production. This pseudo-random number generator code is very much just an experiment and not ready for merging yet. We also block access to /proc/kallsyms for any non-privileged user so that we don't give away our new layout. Does this improve security though? ---------------------------------- The objective of this patch set is to improve a technology that is already merged into the kernel (KASLR). Obviously, this code is not a one stop shopping place for blocking code reuse attacks, but should instead be considered as one of several tools that can be used. In particular, this code is meant to make KASLR more effective in the presence of info leaks. A key point to note is that we are basically accepting that there are many and various ways to leak address today and in the future, and rather than assume that we can stop them all, we should find a method which makes individual info leaks less important. Many people claim that standard KASLR is good enough protection if the attacker does not have access to the host machine, but for example, CVE-2019-0688 demonstrated that addresses can be obtained even with remote attacks. So if you define a threat model in which the kernel has W^X (executable memory is not writable), and ideally XOM (execute only memory, neither readable nor writable), and does have info leaks, this patch will make derandomizing the kernel considerably harder. How much harder will depend on how much entropy we add to the existing entropy of standard KASLR. There are some variables that determine this. Firstly and most obviously, the number of functions you randomize matters. This implementation keeps the existing .text section for code that cannot be randomized - for example, because it was assembly code, or we opted out of randomization for performance reasons. The less sections to randomize, the less entropy. In addition, due to alignment (16 bytes for x86_64), the number of bits in a address that the attacker needs to guess is reduced, as the lower bits are identical. For future work, we could explore randomizing the starting position of the function and padding with INT3s if we wanted to make the lower bits unique. Having a XOM solution for the kernel such as the one for VM guests on X86 platforms that is currently under discussion makes finer grained randomization more effective against JIT-ROP and other variations. Other solutions --------------- CFI is another method of mitigating code reuse attacks. CFI attempts to prevent control flow from being diverted to gadgets (snippets of code anywhere in the text section) by restricting calls to validated function entry points. * Forward Edge CFI Common forward edge CFI implementations will check the function signature to make sure that control flow matches the expected function signature. This reduced the number of calls that will pass a forward edge CFI check to those that match the original function's signature. Unfortunately, the kernel has many functions with the same signature, so forward CFI in and of itself still allows an attacker to potentially change to a different valid function with the same signature. In theory, finer grained randomization can be combined with CFI to make this even harder, so CFI and finer grained KASLR do not need to be competing solutions. For a kernel with both forward edge CFI and function level randomization, an attacker would have to call to a function which not only matched the function signature, but they would also need to take the extra step to find the address of the new function they want to call. In practice, I have not tested this combination, as I used standard kernel build tools (gcc, not clang) and CFI was not an option. * Backward edge CFI Backward edge CFI is not available for X86 at all today, so in the case of modifying a return address, there is no CFI based solution for today's X86 hardware running upstream Linux. Performance Impact ------------------ There are two areas where function reordering can impact performance: boot time latency, and run time performance. * Boot time latency This implementation of finer grained KASLR impacts the boot time of the kernel in several places. It requires additional parsing of the kernel ELF file to obtain the section headers of the sections to be randomized. It calls the random number generator for each section to be randomized to determine that section's new memory location. It copies the decompressed kernel into a new area of memory to avoid corruption when laying out the newly randomized sections. It increases the number of relocations the kernel has to perform at boot time vs. standard KASLR, and it also requires a lookup on each address that needs to be relocated to see if it was in a randomized section and needs to be adjusted by a new offset. Booting a test VM on a modern, well appointed system showed an increase in latency of approximately 1 second. * 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. 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). Image Size ---------- Adding additional section headers as a result of compiling with -ffunction-sections will increase the size of the vmlinux ELF file. In addition, the vmlinux.bin file generated in arch/x86/boot/compressed by objcopy grows significantly with the current POC implementation. This is because the boot heap size must be dramatically increased to support shuffling the sections and re-sorting kallsyms. With a sample kernel compilation using a stock Fedora config, bzImage grew about 7.5X when CONFIG_FG_KASLR was enabled. This is because the boot heap area is included in the image itself. It is possible to mitigate this issue by moving the boot heap out of .bss. Kees Cook has a prototype of this working, and it is included in this patchset. Building -------- To enable fine grained KASLR in the prototype code, you need to have the following config options set (including all the ones you would use to build normal KASLR) CONFIG_FG_KASLR=y Modules ------- The same technique can be applied to modules very easily. This patchset demonstrates how you can randomize modules at load time by shuffling the sections prior to moving them into memory. The module code has some TODOs left in it and has been tested less than the kernel code. References ---------- There are a lot of academic papers which explore finer grained ASLR and CFI. This paper in particular contributed the most to my implementation design as well as my overall understanding of the problem space: Selfrando: Securing the Tor Browser against De-anonymization Exploits, M. Conti, S. Crane, T. Frassetto, et al. For more information on how function layout impacts performance, see: Optimizing Function Placement for Large-Scale Data-Center Applications, G. Ottoni, B. Maher Kees Cook (3): x86/boot: Allow a "silent" kaslr random byte fetch x86/boot/KASLR: Introduce PRNG for faster shuffling x86/boot: Move "boot heap" out of .bss Kristen Carlson Accardi (8): modpost: Support >64K sections x86: tools/relocs: Support >64K section headers x86: Makefile: Add build and config option for CONFIG_FG_KASLR x86: make sure _etext includes function sections x86/tools: Adding relative relocs for randomized functions x86: Add support for finer grained KASLR kallsyms: hide layout and expose seed module: Reorder functions Makefile | 4 + arch/x86/Kconfig | 13 + arch/x86/boot/compressed/Makefile | 8 +- arch/x86/boot/compressed/fgkaslr.c | 751 +++++++++++++++++++++++ arch/x86/boot/compressed/head_32.S | 5 +- arch/x86/boot/compressed/head_64.S | 7 +- arch/x86/boot/compressed/kaslr.c | 6 +- arch/x86/boot/compressed/misc.c | 109 +++- arch/x86/boot/compressed/misc.h | 26 + arch/x86/boot/compressed/vmlinux.lds.S | 1 + arch/x86/boot/compressed/vmlinux.symbols | 15 + arch/x86/include/asm/boot.h | 15 +- arch/x86/include/asm/kaslr.h | 4 +- arch/x86/kernel/vmlinux.lds.S | 18 +- arch/x86/lib/kaslr.c | 83 ++- arch/x86/mm/init.c | 2 +- arch/x86/mm/kaslr.c | 2 +- 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 | 2 +- kernel/kallsyms.c | 30 +- kernel/module.c | 85 +++ scripts/kallsyms.c | 14 +- scripts/link-vmlinux.sh | 4 + scripts/mod/modpost.c | 16 +- 26 files changed, 1312 insertions(+), 70 deletions(-) create mode 100644 arch/x86/boot/compressed/fgkaslr.c create mode 100644 arch/x86/boot/compressed/vmlinux.symbols -- 2.24.1
