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Wed, 24 Jan 2024 22:30:32 -0800 (PST) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id t19-20020a056a00139300b006dd870b51b8sm3201139pfg.126.2024.01.24.22.30.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Jan 2024 22:30:32 -0800 (PST) From: debug@rivosinc.com To: rick.p.edgecombe@intel.com, broonie@kernel.org, Szabolcs.Nagy@arm.com, kito.cheng@sifive.com, keescook@chromium.org, ajones@ventanamicro.com, paul.walmsley@sifive.com, palmer@dabbelt.com, conor.dooley@microchip.com, cleger@rivosinc.com, atishp@atishpatra.org, alex@ghiti.fr, bjorn@rivosinc.com, alexghiti@rivosinc.com Cc: corbet@lwn.net, aou@eecs.berkeley.edu, oleg@redhat.com, akpm@linux-foundation.org, arnd@arndb.de, ebiederm@xmission.com, shuah@kernel.org, brauner@kernel.org, debug@rivosinc.com, guoren@kernel.org, samitolvanen@google.com, evan@rivosinc.com, xiao.w.wang@intel.com, apatel@ventanamicro.com, mchitale@ventanamicro.com, waylingii@gmail.com, greentime.hu@sifive.com, heiko@sntech.de, jszhang@kernel.org, shikemeng@huaweicloud.com, david@redhat.com, charlie@rivosinc.com, panqinglin2020@iscas.ac.cn, willy@infradead.org, vincent.chen@sifive.com, andy.chiu@sifive.com, gerg@kernel.org, jeeheng.sia@starfivetech.com, mason.huo@starfivetech.com, ancientmodern4@gmail.com, mathis.salmen@matsal.de, cuiyunhui@bytedance.com, bhe@redhat.com, chenjiahao16@huawei.com, ruscur@russell.cc, bgray@linux.ibm.com, alx@kernel.org, baruch@tkos.co.il, zhangqing@loongson.cn, catalin.marinas@arm.com, revest@chromium.org, josh@joshtriplett.org, joey.gouly@arm.com, shr@devkernel.io, omosnace@redhat.com, ojeda@kernel.org, jhubbard@nvidia.com, linux-doc@vger.kernel.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: [RFC PATCH v1 26/28] riscv: Documentation for landing pad / indirect branch tracking Date: Wed, 24 Jan 2024 22:21:51 -0800 Message-ID: <20240125062739.1339782-27-debug@rivosinc.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20240125062739.1339782-1-debug@rivosinc.com> References: <20240125062739.1339782-1-debug@rivosinc.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 9579F40021 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: nqhsfqg8reuybtn7o8yj8e93jiz44j9g X-HE-Tag: 1706164233-256539 X-HE-Meta: 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 /xnVzbeo ERgTejmeklGSxSbB1cyUvfqy+b5B5ibsV7Ev4qfbQaZNpDed3MfHTSwpPc1jxKdgP/G1t7TSfUusaz5N5bIxseVMA56ze+YQoJt07lBwTc6llgdF1FddmWszet76zDqTPKUQVorDxJXt/eZmpwtAht0JwiIr7781om+3URV39SHb+hodpvQzaK1l8wuVuamc3UjN8bIu0bdqHKwbGfmwTQZId5GaJlUmqYtXRrk9vnB639syk8mOqrF6h82zszzTX4VbICtOeIAFOq+JJJufLArr8MkPUf9Vv4XYIm2cMC/Ma2TlzROvFRXhmU3OMAVttXQlfy7YGhnhmtaUXZgCqG/HZyvEKoixDm2XHrwbDRQ2gwURKdupmz9ZY8xpFwTVZCAf/KB7MKVhgQkFn/QhjvuKBDPe+vjulFCYfMldINwhK13+caDVqZO56wSuhKJP5Lq4N X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Deepak Gupta 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 + 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` +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 +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. -- 2.43.0