Re: [PATCH v4 28/30] riscv: Documentation for landing pad / indirect branch tracking

Bagas Sanjaya
2024-09-16T02:41:16Z

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On Thu, Sep 12, 2024 at 04:16:47PM -0700, 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.
>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
> Documentation/arch/riscv/zicfilp.rst | 104 +++++++++++++++++++++++++++
> 1 file changed, 104 insertions(+)
> create mode 100644 Documentation/arch/riscv/zicfilp.rst

Don't forget to add toctree entry:

---- >8 ----
diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst
index eecf347ce84944..be7237b6968213 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

> +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.

Sphinx reports new htmldocs warnings:

Documentation/arch/riscv/zicfilp.rst:30: ERROR: Unexpected indentation.
Documentation/arch/riscv/zicfilp.rst:96: ERROR: Unexpected indentation.

I have to fix up the lists:

---- >8 ----
diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst
index 23013ee711ac2c..c0fad1b5caa3d8 100644
--- a/Documentation/arch/riscv/zicfilp.rst
+++ b/Documentation/arch/riscv/zicfilp.rst
@@ -23,22 +23,24 @@ 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
+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)
+- indirect control transfers must land on a landing pad instruction `lpad`.
+ There are two exception to this rule:

- - rs1 = x7. On RISC-V compiler usually does below to reach function
- which is beyond the offset possible J-type instruction.
+ - rs1 = x1 or rs1 = x5, i.e. a return from a function and returns are
+ protected using shadow stack (see zicfiss.rst)

- "auipc x7, <imm>"
- "jalr (x7)"
+ - rs1 = x7. On RISC-V compiler usually does below to reach function
+ which is beyond the offset possible J-type instruction.

- 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.
+ "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
@@ -92,10 +94,11 @@ to lock current settings.
--------------------------------------------------

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`
+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)

> +
> +`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`.
> +

Also inline identifiers/keywords to be consistent with rest of riscv docs:

---- >8 ----
diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst
index c0fad1b5caa3d8..b0a766098f2335 100644
--- a/Documentation/arch/riscv/zicfilp.rst
+++ b/Documentation/arch/riscv/zicfilp.rst
@@ -26,38 +26,38 @@ 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`.
+- 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.
+ which is beyond the offset possible J-type instruction::

- "auipc x7, <imm>"
- "jalr (x7)"
+ 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`.
+``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 with x7. 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
+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.
+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
------------------
@@ -70,25 +70,26 @@ 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
+``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_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_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.
+``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
--------------------------------------------------
@@ -96,12 +97,12 @@ to lock current settings.
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`
+- 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
+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
+Linux kernel will treat this as ``SIGSEV`` with code = ``SEGV_CPERR`` and follow
normal course of signal delivery.

Thanks.

--
An old man doll... just what I always wanted! - Clara

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