[PATCH v3 27/29] riscv: Documentation for landing pad / indirect branch tracking

[Deepak Gupta]

On Fri, May 10, 2024 at 01:30:32PM -0700, Charlie Jenkins wrote:
>On Wed, Apr 03, 2024 at 04:35:15PM -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 at rivosinc.com>
>> ---
>>  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..3007c81f0465
>> --- /dev/null
>> +++ b/Documentation/arch/riscv/zicfilp.rst
>> @@ -0,0 +1,104 @@
>> +.. SPDX-License-Identifier: GPL-2.0
>> +
>> +:Author: Deepak Gupta <debug at 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
>> +
>> +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
>
>What is a return that is not a return from a function?
Those would be a jump or call (depending on convention of whether return is saved in x1/x5)

>
>> +		  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>` and is a HINT nop. `lpad`
>
>I think this should say "x0" or instead of "rd", or mention that rd=x0.

Yeah I missed that. will fix it.

>
>> +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 proglogs can have `lpad` with same
>
>"prologues" instead of "proglogs"

Will fix it.

>
>> +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.
>> --
>> 2.43.2
>>