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AJvYcCVhMa4IUD9UeVVMOuT3qyZZ3WtKrgpoO4k6JQDiNdQCTJJINFsrHuxxgy4CGaChunZoXQi+MkNkWytEq9+sqeZhqOTuz8u3KRrx1A== X-Gm-Message-State: AOJu0YwTUy0shUVsmkd78nAx27f+LbYRIUtn0Tvo76xrPtztrHZ9gGoG rEDfwNGYK+PNcgdIC6rWtp3FksyA3pZV6UVU47wildaqZAj+GsJ9014vLer9y4o= X-Google-Smtp-Source: AGHT+IGgitRfxdN9c/jkn+3dztyByZuJHDxryLBrfrJDLCggaquJnKcgfWgSQwb6qIcM1QjSPmZaxA== X-Received: by 2002:a05:6a20:431a:b0:1aa:43f4:3562 with SMTP id adf61e73a8af0-1afde0a98d8mr11383826637.11.1715620079928; Mon, 13 May 2024 10:07:59 -0700 (PDT) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-6f4d2af2bccsm8222317b3a.170.2024.05.13.10.07.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 May 2024 10:07:59 -0700 (PDT) Date: Mon, 13 May 2024 10:07:54 -0700 From: Deepak Gupta To: Charlie Jenkins Cc: paul.walmsley@sifive.com, rick.p.edgecombe@intel.com, broonie@kernel.org, Szabolcs.Nagy@arm.com, kito.cheng@sifive.com, keescook@chromium.org, ajones@ventanamicro.com, conor.dooley@microchip.com, cleger@rivosinc.com, atishp@atishpatra.org, alex@ghiti.fr, bjorn@rivosinc.com, alexghiti@rivosinc.com, samuel.holland@sifive.com, conor@kernel.org, linux-doc@vger.kernel.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, devicetree@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, corbet@lwn.net, palmer@dabbelt.com, aou@eecs.berkeley.edu, robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org, oleg@redhat.com, akpm@linux-foundation.org, arnd@arndb.de, ebiederm@xmission.com, Liam.Howlett@oracle.com, vbabka@suse.cz, lstoakes@gmail.com, shuah@kernel.org, brauner@kernel.org, andy.chiu@sifive.com, jerry.shih@sifive.com, hankuan.chen@sifive.com, greentime.hu@sifive.com, evan@rivosinc.com, xiao.w.wang@intel.com, apatel@ventanamicro.com, mchitale@ventanamicro.com, dbarboza@ventanamicro.com, sameo@rivosinc.com, shikemeng@huaweicloud.com, willy@infradead.org, vincent.chen@sifive.com, guoren@kernel.org, samitolvanen@google.com, songshuaishuai@tinylab.org, gerg@kernel.org, heiko@sntech.de, bhe@redhat.com, jeeheng.sia@starfivetech.com, cyy@cyyself.name, maskray@google.com, ancientmodern4@gmail.com, mathis.salmen@matsal.de, cuiyunhui@bytedance.com, bgray@linux.ibm.com, mpe@ellerman.id.au, baruch@tkos.co.il, alx@kernel.org, david@redhat.com, catalin.marinas@arm.com, revest@chromium.org, josh@joshtriplett.org, shr@devkernel.io, deller@gmx.de, omosnace@redhat.com, ojeda@kernel.org, jhubbard@nvidia.com Subject: Re: [PATCH v3 27/29] riscv: Documentation for landing pad / indirect branch tracking Message-ID: References: <20240403234054.2020347-1-debug@rivosinc.com> <20240403234054.2020347-28-debug@rivosinc.com> Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: 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 >> --- >> 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 >> +: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, " >> + "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, ` 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 >>