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charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 4BE491C000B X-Stat-Signature: f6em1ckn9851ngu16fsmi73e1tg4q1xp X-HE-Tag: 1715620081-904612 X-HE-Meta: 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 j4cVVVgo 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 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: 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 >>