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Tue, 01 Oct 2024 09:08:24 -0700 (PDT) From: Deepak Gupta Date: Tue, 01 Oct 2024 09:06:37 -0700 Subject: [PATCH 32/33] riscv: Documentation for shadow stack on riscv MIME-Version: 1.0 Message-Id: <20241001-v5_user_cfi_series-v1-32-3ba65b6e550f@rivosinc.com> References: <20241001-v5_user_cfi_series-v1-0-3ba65b6e550f@rivosinc.com> In-Reply-To: <20241001-v5_user_cfi_series-v1-0-3ba65b6e550f@rivosinc.com> To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. 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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 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: 59C9F4001C X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: goy4otczp5p9hw3tdn6gp8yi33h8m65c X-HE-Tag: 1727798906-417835 X-HE-Meta: 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 nshqy2YV p0ngb3dvHsxIIHoB6SQgPaM95oBvVz7YYZJ+n9AP2bxQ6g/3v2o3ZcKSEftMxlkvegCGRrFoPIkiyay9uW1l1v4T6RTmaObRyb8F0Py4WVeGY1N2x3lGnW1aYMgM6unY6NOYBQ8GvvNeBnYHtzXLe3ZcqnyLYXcZjIvuSmO/1Cw6SYWcdZS0uWCOwq9kVNBGuJYVKgpatswtjuQSiRiZWDPVq9FsbFyNvlMrvxN5u4roL5LEU1X1HPB6L95A1wEMlATM+couP6fOrMNFBq9IjEOIfmbV8DB98ewOqC3C8lG7LLRAnKB2xg3xuQ5TTX0MZbRa36OTWhRd1pI/WUQqAe3pTeScc7WmKzStrThGgg9h0Un6EdlV/VsqmR2zt/E/8EqcaIwvFH2iMW+HMqp2RZs1JbNghFrkCyiaC5/YQ8TtksX8c5ZZqPpeQpD2/ZHR2HoZVzS6N7Q6fefM= 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 shadow stack for user mode 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/zicfiss.rst | 176 +++++++++++++++++++++++++++++++++++ 2 files changed, 177 insertions(+) diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst index be7237b69682..e240eb0ceb70 100644 --- a/Documentation/arch/riscv/index.rst +++ b/Documentation/arch/riscv/index.rst @@ -15,6 +15,7 @@ RISC-V architecture vector cmodx zicfilp + zicfiss features diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/riscv/zicfiss.rst new file mode 100644 index 000000000000..5ba389f15b3f --- /dev/null +++ b/Documentation/arch/riscv/zicfiss.rst @@ -0,0 +1,176 @@ +.. SPDX-License-Identifier: GPL-2.0 + +:Author: Deepak Gupta +:Date: 12 January 2024 + +========================================================= +Shadow stack to protect function returns on RISC-V Linux +========================================================= + +This document briefly describes the interface provided to userspace by Linux +to enable shadow stack 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 return addresses present on stack and chain them +together to perform return oriented programming (ROP) and thus compromising +control flow integrity (CFI) of the program. + +Return addresses live on stack and thus 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 ``zicfiss`` extension provides an alternate +stack termed as shadow stack on which return addresses can be safely placed in +prolog of the function and retrieved in epilog. ``zicfiss`` extension makes +following changes: + +- PTE encodings for shadow stack virtual memory + An earlier reserved encoding in first stage translation i.e. + PTE.R=0, PTE.W=1, PTE.X=0 becomes PTE encoding for shadow stack pages. + +- ``sspush x1/x5`` instruction pushes (stores) ``x1/x5`` to shadow stack. + +- ``sspopchk x1/x5`` instruction pops (loads) from shadow stack and compares + with ``x1/x5`` and if un-equal, CPU raises ``software check exception`` with + ``*tval = 3`` + +Compiler toolchain makes sure that function prologue have ``sspush x1/x5`` to +save return address on shadow stack in addition to regular stack. Similarly +function epilogs have ``ld x5, offset(x2)`` followed by ``sspopchk x5`` to +ensure that popped value from regular stack matches with popped value from +shadow stack. + +2. Shadow stack protections and linux memory manager +----------------------------------------------------- + +As mentioned earlier, shadow stack get new page table encodings and thus have +some special properties assigned to them and instructions that operate on them +as below: + +- Regular stores to shadow stack memory raises access store faults. This way + shadow stack memory is protected from stray inadvertant writes. + +- Regular loads to shadow stack memory are allowed. This allows stack trace + utilities or backtrace functions to read true callstack (not tampered). + +- Only shadow stack instructions can generate shadow stack load or shadow stack + store. + +- Shadow stack load / shadow stack store on read-only memory raises AMO/store + page fault. Thus both ``sspush x1/x5`` and ``sspopchk x1/x5`` will raise AMO/ + store page fault. This simplies COW handling in kernel During fork, kernel + can convert shadow stack pages into read-only memory (as it does for regular + read-write memory) and as soon as subsequent ``sspush`` or ``sspopchk`` in + userspace is encountered, then kernel can perform COW. + +- Shadow stack load / shadow stack store on read-write, read-write-execute + memory raises an access fault. This is a fatal condition because shadow stack + should never be operating on read-write, read-write-execute memory. + +3. ELF and psABI +----------------- + +Toolchain sets up :c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property +:c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file. + +4. 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 shadow stack. Thus it's left to dynamic loader to enable +shadow stack for the program. + +5. prctl() enabling +-------------------- + +:c:macro:`PR_SET_SHADOW_STACK_STATUS` / :c:macro:`PR_GET_SHADOW_STACK_STATUS` / +:c:macro:`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow +stack enabling for tasks. prctls are arch agnostic and returns -EINVAL on other +arches. + +* prctl(PR_SET_SHADOW_STACK_STATUS, unsigned long arg) + +If arg1 :c:macro:`PR_SHADOW_STACK_ENABLE` and if CPU supports ``zicfiss`` then +kernel will enable shadow stack for the task. Dynamic loader can issue this +:c:macro:`prctl` once it has determined that all the objects loaded in address +space have support for shadow stack. Additionally if there is a +:c:macro:`dlopen` to an object which wasn't compiled with ``zicfiss``, dynamic +loader can issue this prctl with arg1 set to 0 (i.e. +:c:macro:`PR_SHADOW_STACK_ENABLE` being clear) + +* prctl(PR_GET_SHADOW_STACK_STATUS, unsigned long *arg) + +Returns current status of indirect branch tracking. If enabled it'll return +:c:macro:`PR_SHADOW_STACK_ENABLE`. + +* prctl(PR_LOCK_SHADOW_STACK_STATUS, unsigned long arg) + +Locks current status of shadow stack enabling on the task. User space may want +to run with strict security posture and wouldn't want loading of objects +without ``zicfiss`` support in it and thus would want to disallow disabling of +shadow stack on current task. In that case user space can use this prctl to +lock current settings. + +5. violations related to returns with shadow stack enabled +----------------------------------------------------------- + +Pertaining to shadow stack, CPU raises software check exception in following +condition: + +- On execution of ``sspopchk x1/x5``, ``x1/x5`` didn't match top of shadow + stack. If mismatch happens then cpu does ``*tval = 3`` and raise software + check exception. + +Linux kernel will treat this as :c:macro:`SIGSEV`` with code = +:c:macro:`SEGV_CPERR` and follow normal course of signal delivery. + +6. Shadow stack tokens +----------------------- +Regular stores on shadow stacks are not allowed and thus can't be tampered +with via arbitrary stray writes due to bugs. Method of pivoting / switching to +shadow stack is simply writing to csr ``CSR_SSP`` changes active shadow stack. +This can be problematic because usually value to be written to ``CSR_SSP`` will +be loaded somewhere in writeable memory and thus allows an adversary to +corruption bug in software to pivot to an any address in shadow stack range. +Shadow stack tokens can help mitigate this problem by making sure that: + +- When software is switching away from a shadow stack, shadow stack pointer + should be saved on shadow stack itself and call it ``shadow stack token`` + +- When software is switching to a shadow stack, it should read the + ``shadow stack token`` from shadow stack pointer and verify that + ``shadow stack token`` itself is pointer to shadow stack itself. + +- Once the token verification is done, software can perform the write to + ``CSR_SSP`` to switch shadow stack. + +Here software can be user mode task runtime itself which is managing various +contexts as part of single thread. Software can be kernel as well when kernel +has to deliver a signal to user task and must save shadow stack pointer. Kernel +can perform similar procedure by saving a token on user shadow stack itself. +This way whenever :c:macro:`sigreturn` happens, kernel can read the token and +verify the token and then switch to shadow stack. Using this mechanism, kernel +helps user task so that any corruption issue in user task is not exploited by +adversary by arbitrarily using :c:macro:`sigreturn`. Adversary will have to +make sure that there is a ``shadow stack token`` in addition to invoking +:c:macro:`sigreturn` + +7. Signal shadow stack +----------------------- +Following structure has been added to sigcontext for RISC-V:: + + struct __sc_riscv_cfi_state { + unsigned long ss_ptr; + }; + +As part of signal delivery, shadow stack token is saved on current shadow stack +itself and updated pointer is saved away in :c:macro:`ss_ptr` field in +:c:macro:`__sc_riscv_cfi_state` under :c:macro:`sigcontext`. Existing shadow +stack allocation is used for signal delivery. During :c:macro:`sigreturn`, +kernel will obtain :c:macro:`ss_ptr` from :c:macro:`sigcontext` and verify the +saved token on shadow stack itself and switch shadow stack.