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Date: Wed, 24 Jan 2024 22:21:51 -0800 From: debug@...osinc.com To: rick.p.edgecombe@...el.com, broonie@...nel.org, Szabolcs.Nagy@....com, kito.cheng@...ive.com, keescook@...omium.org, ajones@...tanamicro.com, paul.walmsley@...ive.com, palmer@...belt.com, conor.dooley@...rochip.com, cleger@...osinc.com, atishp@...shpatra.org, alex@...ti.fr, bjorn@...osinc.com, alexghiti@...osinc.com Cc: corbet@....net, aou@...s.berkeley.edu, oleg@...hat.com, akpm@...ux-foundation.org, arnd@...db.de, ebiederm@...ssion.com, shuah@...nel.org, brauner@...nel.org, debug@...osinc.com, guoren@...nel.org, samitolvanen@...gle.com, evan@...osinc.com, xiao.w.wang@...el.com, apatel@...tanamicro.com, mchitale@...tanamicro.com, waylingii@...il.com, greentime.hu@...ive.com, heiko@...ech.de, jszhang@...nel.org, shikemeng@...weicloud.com, david@...hat.com, charlie@...osinc.com, panqinglin2020@...as.ac.cn, willy@...radead.org, vincent.chen@...ive.com, andy.chiu@...ive.com, gerg@...nel.org, jeeheng.sia@...rfivetech.com, mason.huo@...rfivetech.com, ancientmodern4@...il.com, mathis.salmen@...sal.de, cuiyunhui@...edance.com, bhe@...hat.com, chenjiahao16@...wei.com, ruscur@...sell.cc, bgray@...ux.ibm.com, alx@...nel.org, baruch@...s.co.il, zhangqing@...ngson.cn, catalin.marinas@....com, revest@...omium.org, josh@...htriplett.org, joey.gouly@....com, shr@...kernel.io, omosnace@...hat.com, ojeda@...nel.org, jhubbard@...dia.com, linux-doc@...r.kernel.org, linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org Subject: [RFC PATCH v1 26/28] riscv: Documentation for landing pad / indirect branch tracking From: Deepak Gupta <debug@...osinc.com> 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 <debug@...osinc.com> --- 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 <debug@...osinc.com> +: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, <imm>" + "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, <imm_20bit>` 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
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