Exploit Methods/Userspace execution

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Once an attacker has gain control over the instruction pointers, it must be aimed somewhere. The place where attackers have the most control over memory layout tends to be in userspace, so it has been natural to place malicious code in userspace and have the kernel redirection execution there. (Frequently known as "ret2usr".)

For more details, see Userspace access, as that can be superset of userspace execution under some emulation situations.



  • hardware segregation: SMEP (x86), PXN (arm)
  • compiler instrumentation to set high bit on function calls
  • emulate memory segregation via separate page tables (e.g. PAX_MEMORY_UDEREF)

Right now, the upstream options available for Privileged eXecute Never (e.g. PXN, SMEP) are:

CPU Feature Name
ARM v7 (32-bit) non-LPAE CONFIG_CPU_SW_DOMAIN_PAN (since Linux v4.3)
v7 (32-bit) LPAE (e.g. Cortex-A7, A15+) hardware PXN (since Linux v3.19)
v8.0+ (64-bit) hardware PXN
x86 pre-Ivy-Bridge nothing (could use PCID?)
Ivy-Bridge+ (since May 2012) hardware PXN (SMEP)
s/390 hardware PXN (Address Spaces)
powerpc radix MMU (since POWER9) hardware PXN (KUEP, since Linux v4.10)
PPC64 hash MMU (since POWER7) nothing yet, but implementation possible
PPC32 hash MMU (except 601 which doesn't have NX segment) hardware PXN (KUEP)
MPC 8xx hardware PXN (KUEP)
MIPS nothing (could use ASID switching?)