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Tue, 04 Feb 2025 17:22:39 -0800 (PST) From: Deepak Gupta Date: Tue, 04 Feb 2025 17:22:11 -0800 Subject: [PATCH v9 24/26] riscv: Documentation for landing pad / indirect branch tracking MIME-Version: 1.0 Message-Id: <20250204-v5_user_cfi_series-v9-24-b37a49c5205c@rivosinc.com> References: <20250204-v5_user_cfi_series-v9-0-b37a49c5205c@rivosinc.com> In-Reply-To: <20250204-v5_user_cfi_series-v9-0-b37a49c5205c@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 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, Deepak Gupta X-Mailer: b4 0.14.0 X-Rspamd-Queue-Id: D8B8C4000B X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: ywddumzkh4hybe34xyhw33n3iy8gz45m X-HE-Tag: 1738718560-904864 X-HE-Meta: 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 VJubbrZh IifJMOFil68oYCOQjdYmL0lcHZPrt/VyNfSPSvaZLZ0e6UKKntp3e/qJYAAAPvsxcgT8fcuAQwoJ2sSMd1MQf8MNhgxLxY5ztXbm+2ntcvwMj/gXWidtIbOR6WiA/y8Fl9OwC6sFzXMXv76rieH/tv2XbtEw0uWxxa6s4AODkRpSar5fKFtRQRaCYneRzthn5MqKL7wVvYQUGKl711/RFSHq2Xdn5xEDLdMTTFRu72+DZ9Uq281bkvJjArihYGwYMuTHX9vPWVeabYWtxumyj1upEAfY0wmVC42zwGsHjWSFQNUeja32JCxPvVfRFwLH5neO1dKmsyTBqKPTg6BrIS9woG/1GYu+Is+Zui9Odeogf4uvkS4TrN1Xjfz6IRJy/pkjjISyzmKPdMTKMYyhm0Xn11iw4o/6Pzpu4LxexdE+4UBcWB2i7JputdR65JlBFj3kPIHS4eyIiNib0PDNpsDRQdg== 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: 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/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 features diff --git a/Documentation/arch/riscv/zicfilp.rst b/Documentation/arch/riscv/zicfilp.rst new file mode 100644 index 000000000000..a188d78fcde6 --- /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 + +==================================================== +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, `` 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 :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 compiled +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 indirect +branch tracking. prctls are arch agnostic and returns -EINVAL on other arches. + +* 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 enabled indirect branch tracking for the task. Dynamic loader +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 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 :c:macro:`SIGSEV`` with code = +:c:macro:`SEGV_CPERR` and follow normal course of signal delivery.