As I've mentioned before, Android uses Clang's forward-edge Control Flow Integrity (CFI) for the Linux kernel on the Pixel 3 to protect indirect calls from control flow hijacking:
android-developers.googleblog.com/2018/10/contro
This includes support for cross-DSO CFI to support dynamic kernel modules.
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Clang CFI heavily depends on LTO for decent performance and to improve precision beyond types, while also avoiding fundamentally changing the ABI for calls. Cross-DSO (cross-library) CFI is supported but is less efficient and less complete information leads to less precision.
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Android has full support for cross-DSO CFI in userspace and the kernel, including a substantially better userspace implementation than the portable one: struct.github.io/cross_dso_cfi.. This allows the Pixel 3 to support dynamic kernel modules, but it only uses them statically anyway.
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As part of the initial work on Pixel 3 hardening for GrapheneOS, I've migrated to using only static kernel modules. Either way the modules are verified (by both dm-verity and module signing, redundantly), but building them in does reduce attack surface and makes CFI more precise.
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Clang does't yet have backward-edge CFI (clang.llvm.org/docs/ControlFl) to protect return addresses, but Android Q brings Shadow Call Stack support to the kernel (clang.llvm.org/docs/ShadowCal), after the failed attempt at using SafeStack in the kernel for Android P:
android.googlesource.com/kernel/msm/+/f
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Pixel 3 only used a few dynamic modules. There are 8 built from the Qualcomm audio teckpack along with the Qualcomm Atheros driver, seemingly just because it's how those drivers are being developed. The touchscreen driver is also a module since it's different for the 3 and 3 XL.
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I prefer shipping fully static kernels specialized for each device as much as possible for minimal attack surface. I make the same change on the Pixel 2 and 2 XL, which have dynamic modules for Atheros (identical on both) and 4 variant-specific drivers in the standard releases.
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The same concepts apply SELinux policy, which Android splits into a system and vendor policy. Each device ships a specialized vendor policy, even including making lists of permitted ioctl commands for different drivers, like a list of permitted GPU driver commands on the device.
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