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Thu, 24 Apr 2025 00:21:48 -0700 (PDT) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-22db52163d6sm6240765ad.214.2025.04.24.00.21.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Apr 2025 00:21:47 -0700 (PDT) From: Deepak Gupta Date: Thu, 24 Apr 2025 00:20:41 -0700 Subject: [PATCH v13 26/28] riscv: Documentation for landing pad / indirect branch tracking Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20250424-v5_user_cfi_series-v13-26-971437de586a@rivosinc.com> References: <20250424-v5_user_cfi_series-v13-0-971437de586a@rivosinc.com> In-Reply-To: <20250424-v5_user_cfi_series-v13-0-971437de586a@rivosinc.com> To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Lorenzo Stoakes , Paul Walmsley , Palmer Dabbelt , Albert Ou , Conor Dooley , Rob Herring , Krzysztof Kozlowski , Arnd Bergmann , Christian Brauner , Peter Zijlstra , Oleg Nesterov , Eric Biederman , Kees Cook , Jonathan Corbet , Shuah Khan , Jann Horn , Conor Dooley , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, devicetree@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, alistair.francis@wdc.com, richard.henderson@linaro.org, jim.shu@sifive.com, andybnac@gmail.com, kito.cheng@sifive.com, charlie@rivosinc.com, atishp@rivosinc.com, evan@rivosinc.com, cleger@rivosinc.com, alexghiti@rivosinc.com, samitolvanen@google.com, broonie@kernel.org, rick.p.edgecombe@intel.com, rust-for-linux@vger.kernel.org, Zong Li , Deepak Gupta X-Mailer: b4 0.13.0 Adding documentation on landing pad aka indirect branch tracking on riscv and kernel interfaces exposed so that user tasks can enable it. Reviewed-by: Zong Li Signed-off-by: Deepak Gupta --- 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 =20 features =20 diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/risc= v/zicfilp.rst new file mode 100644 index 000000000000..dcf43de73a06 --- /dev/null +++ b/Documentation/arch/riscv/zicfilp.rst @@ -0,0 +1,115 @@ +.. SPDX-License-Identifier: GPL-2.0 + +:Author: Deepak Gupta +:Date: 12 January 2024 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D +Tracking indirect control transfers on RISC-V Linux +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D + +This document briefly describes the interface provided to userspace by Lin= ux +to enable indirect branch tracking for user mode applications on RISC-V + +1. Feature Overview +-------------------- + +Memory corruption issues usually result into crashes, however when in hand= s of +an adversary and if used creatively can result into a variety security iss= ues. + +One of those security issues can be code re-use attacks on program where a= dversary +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 co= rruption +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 =3D x1 or rs1 =3D x5, i.e. a return from a function and returns are + protected using shadow stack (see zicfiss.rst) + + - rs1 =3D x7. On RISC-V compiler usually does below to reach function + which is beyond the offset possible J-type instruction:: + + auipc x7, + jalr (x7) + + Such form of indirect control transfer are still immutable and don't rely + on memory and thus rs1=3Dx7 is exempted from tracking and considered s= oftware + guarded jumps. + +``lpad`` instruction is pseudo of ``auipc rd, `` with ``rd=3Dx0= `` and +is a HINT nop. ``lpad`` instruction must be aligned on 4 byte boundary and +compares 20 bit immediate with x7. If ``imm_20bit`` =3D=3D 0, CPU doesn't = perform +any comparision with ``x7``. If ``imm_20bit`` !=3D 0, then ``imm_20bit`` m= ust +match ``x7`` else CPU will raise ``software check exception`` (``cause=3D1= 8``) +with ``*tval =3D 2``. + +Compiler can generate a hash over function signatures and setup them (trun= cated +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 comp= iled +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 ind= irect +branch tracking. prctls are arch agnostic and returns -EINVAL on other arc= hes. + +* 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 enable indirect branch tracking for the task. Dynamic loa= der +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 m= ay +want to run with strict security posture and wouldn't want loading of obje= cts +without ``zicfilp`` support in it and thus would want to disallow disablin= g of +indirect branch tracking. In that case user space can use this prctl to lo= ck +current settings. + +5. violations related to indirect branch tracking +-------------------------------------------------- + +Pertaining to indirect branch tracking, CPU raises software check exceptio= n 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 =3D 2`` is captured and software check exception is +raised (``cause=3D18``) + +Linux kernel will treat this as :c:macro:`SIGSEV`` with code =3D +:c:macro:`SEGV_CPERR` and follow normal course of signal delivery. --=20 2.43.0