From nobody Tue Oct 7 09:00:14 2025 Received: from mail-pf1-f175.google.com (mail-pf1-f175.google.com [209.85.210.175]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 47F8F2FEE3B for ; Fri, 11 Jul 2025 19:47:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.175 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752263253; cv=none; b=lUlx0Tt+XNGFFtMwkg5xYC8ZJLL4B3ZyuxYz8oALp7ij8tCHU0f/Bm6z47YlRaDaif6i5fO4OaxazMVG+bYvMBFfFn2S3f2WFg4P8ap+wI39N92chf3c+FMDAp8wSD1NK6c/7DkxRskl2b1GSDzDyKMIpqIyZI59OnTxXEwUXlI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752263253; c=relaxed/simple; bh=rpxI7deGxR1b1p2DEnatELgrO8HyrKziLzhoGXBzZ0c=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=P0Lvq3YTh4J9Q4koXjIlTKWHMuU7cZhTbDUEPxqr2rBmlq8p/U6P+8kxiMJW0pHETI2QUb5SlbRo9vQhFu6ADXHeFDcgac1aOJpXIbl235VRByQ/2g4mIN77M2dUbUlCYl/U8yf7nuVeJAPtjE6p6MC8Kj6i7zqaGABbiJcEtbk= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=rivosinc.com; spf=pass smtp.mailfrom=rivosinc.com; dkim=pass (2048-bit key) header.d=rivosinc-com.20230601.gappssmtp.com header.i=@rivosinc-com.20230601.gappssmtp.com header.b=FnVvoZmo; arc=none smtp.client-ip=209.85.210.175 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=rivosinc.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=rivosinc.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=rivosinc-com.20230601.gappssmtp.com header.i=@rivosinc-com.20230601.gappssmtp.com header.b="FnVvoZmo" Received: by mail-pf1-f175.google.com with SMTP id d2e1a72fcca58-7490702fc7cso1625855b3a.1 for ; Fri, 11 Jul 2025 12:47:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=rivosinc-com.20230601.gappssmtp.com; s=20230601; t=1752263251; x=1752868051; darn=vger.kernel.org; h=cc:to:in-reply-to:references:message-id:content-transfer-encoding :mime-version:subject:date:from:from:to:cc:subject:date:message-id :reply-to; bh=04baGaWIOcpAM10BsvDKc2YnzV44Y+9Sv7IQZJnFcq0=; b=FnVvoZmoarFmrHbELpKZcvglhvflm90C2qqxKP9FUXnEAMjQtjo/YiT9AyxA2EC/Ub WU9eZqAG4OWUxkYmuiJ82axCrcQywpmhCg1BGkqRNK0oHjKG0gHl0ngQgOb0tPMUJgZv updpf21JiaJEVADHZe7a/z70U3WQHYX1ttqAbCv9IYGGpAvTz7UWhL2Oz7DCqCc1UFp1 cH88V9gw+WaJGdkcehT9r1Mo+FO4pagUErr1MUAjp+n/t6d5YfjnPViZ5pSe9+kvUvp4 hn/F8EVwVf67BfIYonKtTwxOWVUM51TyBRl/83n76zKtQkLHz0hGT78AcvyR7OzmRrFk itPw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1752263251; x=1752868051; h=cc:to:in-reply-to:references:message-id:content-transfer-encoding :mime-version:subject:date:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=04baGaWIOcpAM10BsvDKc2YnzV44Y+9Sv7IQZJnFcq0=; b=O/CqHL4lz0euazpKpI+Ug6Eu8v3YEeOR3iG/3rJse+iOl17LYT70/BJVJ222BiiiIf InpEqntjU3u5I0700jrkWuDEkcPtOseMG6vJifrDu8FiK+IJuCiXvJHsBuy8Kj1abNKl RN5QzoCqh6F7Z2ebG95LU20vvuzUNAxsVn4EtBAQ70HUZVaXGoaXOqgWYj7U7TqEqZyl 1v7HtEO3LuApUk1Rg3oRU1npDc4Jl9vc6k8FNB64RKhbwDgZrHf47JWwW+mbBRPzYgYZ XvEMjDcd0uBuaiG8vLWTIDs81ED+OcckqyZZx6XfUY9s0QKvau66/yoG0AhQVjjfq2tr mFMw== X-Gm-Message-State: AOJu0YzobFl2TSMUlVyeqD5G6lszmcRUgCVe4tEmSCwQvbF7VXC6wmBk zEqK0gcA4s6Q1kwS6r+BjfXlHrAU+g7E7TcDkFkBAgrEBpS0YUzQbZyCUI2cYNVrGgY= X-Gm-Gg: ASbGncscCAoj+nVPPWUAbNSAlvrIdLbzujtgJKhsR9jRomo9PMES1cDmUkE03CDFAqX /7oz7ExvA++x0swi0ax9Cg81mcsClcwyG5FcH8+T5dDJuV0Oso1LNVPGpGugEJZ1V62ZHWV+zBy OHzEWoZgUvq4JGsbHXrmpNG6eN+Cl06QRhFRYCVO3Hfuwt3M+vNdRvhJJdYvsg7xLJeIoQAT3tI WSMLhwipOF+hLzwpL+9jxE/mpTRJjVBLkl6gIp5z8GBZ9t0PlCHMuqcTQHCmYaR0nu7kO08eq7s fAOm+MCGr7Rg1COsDvFrJPKzshk072/OR6fLVdgYKfRXcABU52Cw2KAaldLJ3lme1wQMP23BdoV PtTMyWDLSSexuCgSA9aEPKTZnbjiD2dpI X-Google-Smtp-Source: AGHT+IHsGnXWVV+ma+hIdvW5ynvznV2fkGjoqBcWdZaSS9O1QOkC3QnmVGpKX/3rJyzeOa1af9XFkw== X-Received: by 2002:a05:6a00:23c8:b0:748:f41d:69d2 with SMTP id d2e1a72fcca58-74f1beecbd7mr5517197b3a.4.1752263250532; Fri, 11 Jul 2025 12:47:30 -0700 (PDT) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-74eb9e06995sm5840977b3a.38.2025.07.11.12.47.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Jul 2025 12:47:30 -0700 (PDT) From: Deepak Gupta Date: Fri, 11 Jul 2025 12:46:30 -0700 Subject: [PATCH v18 25/27] 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: <20250711-v5_user_cfi_series-v18-25-a8ee62f9f38e@rivosinc.com> References: <20250711-v5_user_cfi_series-v18-0-a8ee62f9f38e@rivosinc.com> In-Reply-To: <20250711-v5_user_cfi_series-v18-0-a8ee62f9f38e@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?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Benno Lossin 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