Google is committed to advancing racial equity for Black communities. See how.

Kernel Hardening

Android 8.0 added kernel hardening features to help mitigate kernel vulnerabilities and find bugs in kernel drivers. The features are in kernel/common in branches android-3.18, android-4.4, and android-4.9.


To acquire these features, device manufacturers and SOCs should merge all hardening patches from kernel/common to their kernel tree and enable the following kernel configuration options:

  • Hardened usercopy: CONFIG_HARDENED_USERCOPY=y
  • PAN emulation - arm64: CONFIG_ARM64_SW_TTBR0_PAN=y
  • PAN emulation - arm: CONFIG_CPU_SW_DOMAIN_PAN=y
  • KASLR - 4.4 and later kernels: CONFIG_RANDOMIZE_BASE=y

KASLR also requires bootloader support for passing hardware entropy through either the device tree node /chosen/kaslr-seed or by implementing EFI_RNG_PROTOCOL.

Also ensure existing hardening features are enabled:

  • Stack buffer overflow mitigation: CONFIG_CC_STACKPROTECTOR_STRONG=y
  • Internal memory protection: CONFIG_DEBUG_RODATA=y or CONFIG_STRICT_KERNEL_RWX=y
  • Restrict user-space access from kernel - x86 (enabled by default): CONFIG_X86_SMAP=y


To test your implementation, add CONFIG_LKDTM=y to the kernel configuration and confirm that each of the following commands lead to a kernel panic:

echo ACCESS_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
echo WRITE_RO > /sys/kernel/debug/provoke-crash/DIRECT
echo WRITE_RO_AFTER_INIT > /sys/kernel/debug/provoke-crash/DIRECT
echo WRITE_KERN > /sys/kernel/debug/provoke-crash/DIRECT
echo EXEC_STACK > /sys/kernel/debug/provoke-crash/DIRECT
echo EXEC_RODATA > /sys/kernel/debug/provoke-crash/DIRECT
echo EXEC_KMALLOC > /sys/kernel/debug/provoke-crash/DIRECT
echo EXEC_VMALLOC > /sys/kernel/debug/provoke-crash/DIRECT
echo CORRUPT_STACK > /sys/kernel/debug/provoke-crash/DIRECT

For android-4.9:

echo USERCOPY_HEAP_SIZE_TO > /sys/kernel/debug/provoke-crash/DIRECT
echo USERCOPY_HEAP_SIZE_FROM > /sys/kernel/debug/provoke-crash/DIRECT

Common issues

These changes are likely to expose bugs in kernel drivers, which need to be fixed either by the device manufacturer or the owner of the kernel driver.

  • Hardened usercopy exposes incorrect bounds checking when copying data to/from user space. These should be fixed like any other memory corruption bugs.
  • PAN emulation exposes direct user space access from the kernel, which is not allowed. Drivers attempting to access user space memory need to be changed to use the standard copy_to_user()/copy_from_user() functions instead.