Tracking indirect control transfers on RISC-V Linux

This document briefly describes the interface provided to userspace by Linux to enable indirect branch tracking for user mode applications on RISC-V.

1. Feature Overview

Memory corruption issues usually result in crashes. However, in the hands of a creative adversary, these can result in a variety of security issues.

Some of those security issues can be code re-use attacks, where an adversary can use corrupt function pointers, chaining them together to perform jump oriented programming (JOP) or call oriented programming (COP) and thus compromise control flow integrity (CFI) of the program.

Function pointers live in read-write memory and thus are susceptible to corruption. This can allow an adversary to control the program counter (PC) value. On RISC-V, the zicfilp extension enforces a restriction on such indirect control transfers:

  • Indirect control transfers must land on a landing pad instruction lpad. There are two exceptions to this rule:

    • rs1 = x1 or rs1 = x5, i.e. a return from a function and returns are protected using shadow stack (see Shadow stack to protect function returns on RISC-V Linux)

    • rs1 = x7. On RISC-V, the compiler usually does the following to reach a function which is beyond the offset of possible J-type instruction:

      auipc x7, <imm>
jalr (x7)

      This form of indirect control transfer is immutable and doesn’t rely on memory. Thus rs1=x7 is exempted from tracking and these are considered software guarded jumps.

The lpad instruction is a pseudo-op of auipc rd, <imm_20bit> with rd=x0. This is a HINT op. The lpad instruction must be aligned on a 4 byte boundary. It compares the 20 bit immediate with x7. If imm_20bit == 0, the CPU doesn’t perform any comparison with x7. If imm_20bit != 0, then imm_20bit must match x7 else CPU will raise software check exception (cause=18) with *tval = 2.

The compiler can generate a hash over function signatures and set them up (truncated to 20 bits) in x7 at callsites. Function prologues can have lpad instructions encoded with the same function hash. This further reduces the number of valid program counter addresses a call site can reach.

2. ELF and psABI

The toolchain sets up GNU_PROPERTY_RISCV_FEATURE_1_FCFI for property GNU_PROPERTY_RISCV_FEATURE_1_AND in the notes section of the object file.

3. Linux enabling

User space programs can have multiple shared objects loaded in their address spaces. It’s a difficult task to make sure all the dependencies have been compiled with indirect branch support. Thus it’s left to the dynamic loader to enable indirect branch tracking for the program.

4. prctl() enabling

PR_SET_INDIR_BR_LP_STATUS / PR_GET_INDIR_BR_LP_STATUS / PR_LOCK_INDIR_BR_LP_STATUS are three prctls added to manage indirect branch tracking. These prctls are architecture-agnostic and return -EINVAL if the underlying functionality is not supported.

  • prctl(PR_SET_INDIR_BR_LP_STATUS, unsigned long arg)

If arg1 is PR_INDIR_BR_LP_ENABLE and if CPU supports zicfilp then the kernel will enable indirect branch tracking for the task. The dynamic loader can issue this prctl once it has determined that all the objects loaded in the address space support indirect branch tracking. Additionally, if there is a dlopen to an object which wasn’t compiled with zicfilp, the dynamic loader can issue this prctl with arg1 set to 0 (i.e. PR_INDIR_BR_LP_ENABLE cleared).

  • prctl(PR_GET_INDIR_BR_LP_STATUS, unsigned long * arg)

Returns the current status of indirect branch tracking. If enabled it’ll return PR_INDIR_BR_LP_ENABLE

  • prctl(PR_LOCK_INDIR_BR_LP_STATUS, unsigned long arg)

Locks the current status of indirect branch tracking on the task. User space may want to run with a strict security posture and wouldn’t want loading of objects without zicfilp support in them, to disallow disabling of indirect branch tracking. In this case, user space can use this prctl to lock the current settings.