| Message ID | 20240912231650.3740732-29-debug@rivosinc.com |
|---|---|
| State | New |
| Headers | show |
| Series | riscv control-flow integrity for usermode | expand |
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.
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@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 + 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.
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