Re: [PATCH v12 26/28] riscv: Documentation for landing pad / indirect branch tracking

Alexandre Ghiti
2025-04-08T08:37:11Z

On 14/03/2025 22:39, Deepak Gupta wrote:
> Adding documentation on landing pad aka indirect branch tracking on riscv
> and kernel interfaces exposed so that user tasks can enable it.
>
> Reviewed-by: Zong Li <zong.li@sifive.com>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
> Documentation/arch/riscv/index.rst | 1 +
> Documentation/arch/riscv/zicfilp.rst | 115 +++++++++++++++++++++++++++++++++++
> 2 files changed, 116 insertions(+)
>
> diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst
> index eecf347ce849..be7237b69682 100644
> --- a/Documentation/arch/riscv/index.rst
> +++ b/Documentation/arch/riscv/index.rst
> @@ -14,6 +14,7 @@ RISC-V architecture
> uabi
> vector
> cmodx
> + zicfilp
>
> features
>
> diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst
> new file mode 100644
> index 000000000000..a188d78fcde6
> --- /dev/null
> +++ b/Documentation/arch/riscv/zicfilp.rst
> @@ -0,0 +1,115 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +:Author: Deepak Gupta <debug@rivosinc.com>
> +:Date: 12 January 2024
> +
> +====================================================
> +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 RISV-V

s/RISV-V/RISC-V and add a dot at the end of the line.

> +
> +1. Feature Overview
> +--------------------
> +
> +Memory corruption issues usually result in to crashes, however when in hands of

s/in to/into?

> +an adversary and if used creatively can result into variety security issues.

"into a variety of security issues" ?

> +
> +One of those security issues can be code re-use attacks on program where adversary
> +can use corrupt function pointers and chain them together to perform jump oriented
> +programming (JOP) or call oriented programming (COP) and thus compromising control
> +flow integrity (CFI) of the program.
> +
> +Function pointers live in read-write memory and thus are susceptible to corruption
> +and allows an adversary to reach any program counter (PC) in address space. On
> +RISC-V zicfilp extension enforces a restriction on such indirect control
> +transfers:
> +
> +- indirect control transfers must land on a landing pad instruction ``lpad``.
> + There are two exception to this rule:
> +
> + - rs1 = x1 or rs1 = x5, i.e. a return from a function and returns are
> + protected using shadow stack (see zicfiss.rst)
> +
> + - rs1 = x7. On RISC-V compiler usually does below to reach function
> + which is beyond the offset possible J-type instruction::
> +
> + auipc x7, <imm>
> + jalr (x7)
> +
> + Such form of indirect control transfer are still immutable and don't rely
> + on memory and thus rs1=x7 is exempted from tracking and considered software
> + guarded jumps.
> +
> +``lpad`` instruction is pseudo of ``auipc rd, <imm_20bit>`` with ``rd=x0`` and
> +is a HINT nop. ``lpad`` instruction must be aligned on 4 byte boundary and
> +compares 20 bit immediate withx7. If ``imm_20bit`` == 0, CPU don't perform any

s/withx7/with x7

s/don't/doesn't

> +comparision with ``x7``. If ``imm_20bit`` != 0, then ``imm_20bit`` must match
> +``x7`` else CPU will raise ``software check exception`` (``cause=18``) with
> +``*tval = 2``.
> +
> +Compiler can generate a hash over function signatures and setup them (truncated
> +to 20bit) in x7 at callsites and function prologues can have ``lpad`` with same
> +function hash. This further reduces number of program counters a call site can
> +reach.
> +
> +2. ELF and psABI
> +-----------------
> +
> +Toolchain sets up :c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_FCFI` for property
> +:c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file.
> +
> +3. Linux enabling
> +------------------
> +
> +User space programs can have multiple shared objects loaded in its address space
> +and it's a difficult task to make sure all the dependencies have been compiled
> +with support of indirect branch. Thus it's left to dynamic loader to enable
> +indirect branch tracking for the program.
> +
> +4. prctl() enabling
> +--------------------
> +
> +:c:macro:`PR_SET_INDIR_BR_LP_STATUS` / :c:macro:`PR_GET_INDIR_BR_LP_STATUS` /
> +:c:macro:`PR_LOCK_INDIR_BR_LP_STATUS` are three prctls added to manage indirect
> +branch tracking. prctls are arch agnostic and returns -EINVAL on other arches.
> +
> +* prctl(PR_SET_INDIR_BR_LP_STATUS, unsigned long arg)
> +
> +If arg1 is :c:macro:`PR_INDIR_BR_LP_ENABLE` and if CPU supports ``zicfilp``
> +then kernel will enabled indirect branch tracking for the task. Dynamic loader

s/will enabled/will enable

> +can issue this :c:macro:`prctl` once it has determined that all the objects
> +loaded in address space support indirect branch tracking. Additionally if there
> +is a `dlopen` to an object which wasn't compiled with ``zicfilp``, dynamic
> +loader can issue this prctl with arg1 set to 0 (i.e.
> +:c:macro:`PR_INDIR_BR_LP_ENABLE` being clear)
> +
> +* prctl(PR_GET_INDIR_BR_LP_STATUS, unsigned long arg)
> +
> +Returns current status of indirect branch tracking. If enabled it'll return
> +:c:macro:`PR_INDIR_BR_LP_ENABLE`
> +
> +* prctl(PR_LOCK_INDIR_BR_LP_STATUS, unsigned long arg)
> +
> +Locks current status of indirect branch tracking on the task. User space may
> +want to run with strict security posture and wouldn't want loading of objects
> +without ``zicfilp`` support in it and thus would want to disallow disabling of
> +indirect branch tracking. In that case user space can use this prctl to lock
> +current settings.
> +
> +5. violations related to indirect branch tracking
> +--------------------------------------------------
> +
> +Pertaining to indirect branch tracking, CPU raises software check exception in
> +following conditions:
> +
> +- missing ``lpad`` after indirect call / jmp
> +- ``lpad`` not on 4 byte boundary
> +- ``imm_20bit`` embedded in ``lpad`` instruction doesn't match with ``x7``
> +
> +In all 3 cases, ``*tval = 2`` is captured and software check exception is
> +raised (``cause=18``)
> +
> +Linux kernel will treat this as :c:macro:`SIGSEV`` with code =
> +:c:macro:`SEGV_CPERR` and follow normal course of signal delivery.
>