[PATCH v4 28/30] 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/zicfilp.rst | 104 +++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 Documentation/arch/riscv/zicfilp.rst

diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst
new file mode 100644
index 000000000000..23013ee711ac
--- /dev/null
+++ b/Documentation/arch/riscv/zicfilp.rst
@@ -0,0 +1,104 @@
+.. 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 `GNU_PROPERTY_RISCV_FEATURE_1_FCFI` for property
+`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
+--------------------
+
+`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. prctls are arch agnostic and returns -EINVAL on other arches.
+
+`PR_SET_INDIR_BR_LP_STATUS`: If arg1 `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 `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. `PR_INDIR_BR_LP_ENABLE` being clear)
+
+`PR_GET_INDIR_BR_LP_STATUS`: Returns current status of indirect branch tracking.
+If enabled it'll return `PR_INDIR_BR_LP_ENABLE`
+
+`PR_LOCK_INDIR_BR_LP_STATUS`: 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 `SIGSEV`` with code = `SEGV_CPERR` and follow
+normal course of signal delivery.
-- 
2.45.0