[PATCH v6 31/33] riscv: Documentation for landing pad / indirect branch tracking

[Deepak Gupta]

Adding documentation on landing pad aka indirect branch tracking on riscv
and kernel interfaces exposed so that user tasks can enable it.

Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
---
 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@xxxxxxxxxxxx>
+: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
+
+1. Feature Overview
+--------------------
+
+Memory corruption issues usually result in to crashes, however when in hands of
+an adversary and if used creatively can result into variety 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
+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
+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.

-- 
2.45.0