codeblog code is freedom — patching my itch


security things in Linux v4.7

Filed under: Chrome OS,Debian,Kernel,Security,Ubuntu,Ubuntu-Server — kees @ 12:47 am

Previously: v4.6. Onward to security things I found interesting in Linux v4.7:

KASLR text base offset for MIPS

Matt Redfearn added text base address KASLR to MIPS, similar to what’s available on x86 and arm64. As done with x86, MIPS attempts to gather entropy from various build-time, run-time, and CPU locations in an effort to find reasonable sources during early-boot. MIPS doesn’t yet have anything as strong as x86’s RDRAND (though most have an instruction counter like x86’s RDTSC), but it does have the benefit of being able to use Device Tree (i.e. the “/chosen/kaslr-seed” property) like arm64 does. By my understanding, even without Device Tree, MIPS KASLR entropy should be as strong as pre-RDRAND x86 entropy, which is more than sufficient for what is, similar to x86, not a huge KASLR range anyway: default 8 bits (a span of 16MB with 64KB alignment), though CONFIG_RANDOMIZE_BASE_MAX_OFFSET can be tuned to the device’s memory, giving a maximum of 11 bits on 32-bit, and 15 bits on EVA or 64-bit.

SLAB freelist ASLR

Thomas Garnier added CONFIG_SLAB_FREELIST_RANDOM to make slab allocation layouts less deterministic with a per-boot randomized freelist order. This raises the bar for successful kernel slab attacks. Attackers will need to either find additional bugs to help leak slab layout information or will need to perform more complex grooming during an attack. Thomas wrote a post describing the feature in more detail here: Randomizing the Linux kernel heap freelists. (SLAB is done in v4.7, and SLUB in v4.8.)

eBPF JIT constant blinding

Daniel Borkmann implemented constant blinding in the eBPF JIT subsystem. With strong kernel memory protections (CONFIG_DEBUG_RODATA) in place, and with the segregation of user-space memory execution from kernel (i.e SMEP, PXN, CONFIG_CPU_SW_DOMAIN_PAN), having a place where user-space can inject content into an executable area of kernel memory becomes very high-value to an attacker. The eBPF JIT was exactly such a thing: the use of BPF constants could result in the JIT producing instruction flows that could include attacker-controlled instructions (e.g. by directing execution into the middle of an instruction with a constant that would be interpreted as a native instruction). The eBPF JIT already uses a number of other defensive tricks (e.g. random starting position), but this added randomized blinding to any BPF constants, which makes building a malicious execution path in the eBPF JIT memory much more difficult (and helps block attempts at JIT spraying to bypass other protections).

Elena Reshetova updated a 2012 proof-of-concept attack to succeed against modern kernels to help provide a working example of what needed fixing in the JIT. This serves as a thorough regression test for the protection.

The cBPF JITs that exist in ARM, MIPS, PowerPC, and Sparc still need to be updated to eBPF, but when they do, they’ll gain all these protections immediatley.

Bottom line is that if you enable the (disabled-by-default) bpf_jit_enable sysctl, be sure to set the bpf_jit_harden sysctl to 2 (to perform blinding even for root).

fix brk ASLR weakness on arm64 compat

There have been a few ASLR fixes recently (e.g. ET_DYN, x86 32-bit unlimited stack), and while reviewing some suggested fixes to arm64 brk ASLR code from Jon Medhurst, I noticed that arm64’s brk ASLR entropy was slightly too low (less than 1 bit) for 64-bit and noticeably lower (by 2 bits) for 32-bit compat processes when compared to native 32-bit arm. I simplified the code by using literals for the entropy. Maybe we can add a sysctl some day to control brk ASLR entropy like was done for mmap ASLR entropy.

LoadPin LSM

LSM stacking is well-defined since v4.2, so I finally upstreamed a “small” LSM that implements a protection I wrote for Chrome OS several years back. On systems with a static root of trust that extends to the filesystem level (e.g. Chrome OS’s coreboot+depthcharge boot firmware chaining to dm-verity, or a system booting from read-only media), it’s redundant to sign kernel modules (you’ve already got the modules on read-only media: they can’t change). The kernel just needs to know they’re all coming from the correct location. (And this solves loading known-good firmware too, since there is no convention for signed firmware in the kernel yet.) LoadPin requires that all modules, firmware, etc come from the same mount (and assumes that the first loaded file defines which mount is “correct”, hence load “pinning”).

That’s it for v4.7. Prepare yourself for v4.8 next!

© 2016, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 License.
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