From nobody Sat Nov 23 20:59:02 2024 Received: from mail-pl1-f169.google.com (mail-pl1-f169.google.com [209.85.214.169]) (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 420271E9098 for ; Mon, 11 Nov 2024 20:55:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.169 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731358514; cv=none; b=MkC5RDaHH75xZkn3is502ejgEdVVM1V0bBqfVzjw2nEgEqXIyTZyrBOCxEJH2BTVo+qsxLb/BH+X11DQUOzBYmANdBbkTSXDTTe0K3yK6E0ys2TOaxi3wseUbiO7mcT5TCqlpZzNQ5xeNg+6rtVJklLit8RC8fEexNwZWAD086c= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731358514; c=relaxed/simple; bh=6Kpf3Jg7E+DUwQJuMhkUpYlGBvZwuWthU70dbXIzd3Q=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=qukedpaHqXSSFNem4YbITfNjrALgzHTh+1ES5tWldV30Ron1NSwcEpW61k3+ul+oGpg5/tFtFUtX3JyzAy1U/mh6CnKToILFJ392po7UzAjjb7y7/FPo3lqcsOhwOT23bE/AS1Li+xIC2WBWiCy42ynbNylubdKjXO3eIN6LleY= 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=T5lHeQga; arc=none smtp.client-ip=209.85.214.169 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="T5lHeQga" Received: by mail-pl1-f169.google.com with SMTP id d9443c01a7336-20ce5e3b116so43986245ad.1 for ; Mon, 11 Nov 2024 12:55:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=rivosinc-com.20230601.gappssmtp.com; s=20230601; t=1731358512; x=1731963312; 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=NYP5wvilXBl9e2AdD/12DI5Fd1eAjNDT/0JkZD4bKgk=; b=T5lHeQgaUCs1kUpkZ1jnUWyswPlCIFdB0GelT9GqrQPirhSR77lAu33fbntxYHbCwe qmxwTB0apGIIJORHnHRPMld6WRCLHYjEwsm97xyA8vl00AVXnGEtd4G847lV0jdyo81v zD/QOXcdI+mHJTURt9QwOhoOZCAxJkRGXLHi/Ei5+oTjqF16fTl34CNR3JXCyraGk2bW ZK0ZuByyXxsoH8CucrRsexGsPcEgUwIAcfy87ruGKje7EF68lkKGXfcxt4pkIF1/Apqy kB/wjL4YvGdJTTV1GGjnbq8TfWUThOzQJdXji931YwjUvVGtEm5Ji+BRSgFDS5pHpkfP 1xVQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1731358512; x=1731963312; 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=NYP5wvilXBl9e2AdD/12DI5Fd1eAjNDT/0JkZD4bKgk=; b=C8iHPK6wjarAI6gyG61p7ORmi9aWbw+gzvgV0JSnUXA9ZRG/Bs1IN/imXAXn+ADewl gN2NznQNDgtJYMjGqm5keGfRor8ZKQOuCITcriBCz7JxfVJ3KEA8jyiyrKk5QOZswQnb /GzSz4kOD9ElH/pYuLMs9nZNCT6uUYD0bBsaVuo9T+GhanwvRNaR6KnTCsNyIxVDTMe5 JziObFKgoJYZgY6mQv4aVfSjS85q9jmy6E4keiSNWlbCFksnq8zk1Rwr5VMxFQVlWNdl 3aQS1RyGoDhfF7T2OxqXVG5NbiMdNKRylDc57DDnGn85bLE367UStAHBpF56SfychBu+ BkqA== X-Gm-Message-State: AOJu0YyP2g3cX3G5iJ5jq8AoRj9NmKPQI7Yn0d00ExQC6h1maft/NVMJ zKilYoVCykn6gnzdBV6rJQH9ecXHLKw9sxPF3y/EDFeqsul55EyWpn2vcxF8/0Y= X-Google-Smtp-Source: AGHT+IGvZ1Qr8oDf21hnmw+UY/eeYz7wQwk8M8v9lcOAYNLAnYOjMSOnfptGUiFBUzt6eZ3xPP39jw== X-Received: by 2002:a17:902:db12:b0:211:6b25:d824 with SMTP id d9443c01a7336-21183e11eacmr202524465ad.35.1731358511517; Mon, 11 Nov 2024 12:55:11 -0800 (PST) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2e9a5fd1534sm9059974a91.42.2024.11.11.12.55.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 Nov 2024 12:55:11 -0800 (PST) From: Deepak Gupta Date: Mon, 11 Nov 2024 12:54:13 -0800 Subject: [PATCH v8 28/29] riscv: Documentation for shadow stack on riscv 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: <20241111-v5_user_cfi_series-v8-28-dce14aa30207@rivosinc.com> References: <20241111-v5_user_cfi_series-v8-0-dce14aa30207@rivosinc.com> In-Reply-To: <20241111-v5_user_cfi_series-v8-0-dce14aa30207@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 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 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 =20 features =20 diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/risc= v/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 + +=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=3D=3D=3D=3D=3D +Shadow stack to protect function returns 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=3D=3D=3D=3D=3D + +This document briefly describes the interface provided to userspace by Lin= ux +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 han= ds of +an adversary and if used creatively can result into variety security issue= s. + +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 cou= nter +(PC) in address space. On RISC-V ``zicfiss`` extension provides an alterna= te +stack termed as shadow stack on which return addresses can be safely place= d 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=3D0, PTE.W=3D1, PTE.X=3D0 becomes PTE encoding for shadow stack p= ages. + +- ``sspush x1/x5`` instruction pushes (stores) ``x1/x5`` to shadow stack. + +- ``sspopchk x1/x5`` instruction pops (loads) from shadow stack and compar= es + with ``x1/x5`` and if un-equal, CPU raises ``software check exception`` = with + ``*tval =3D 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 h= ave +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 w= ay + 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/st= ore + 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, kern= el + can convert shadow stack pages into read-only memory (as it does for reg= ular + 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 comp= iled +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_STAT= US` / +:c:macro:`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage sh= adow +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 add= ress +space have support for shadow stack. Additionally if there is a +:c:macro:`dlopen` to an object which wasn't compiled with ``zicfiss``, dyn= amic +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 disablin= g 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 followi= ng +condition: + +- On execution of ``sspopchk x1/x5``, ``x1/x5`` didn't match top of shadow + stack. If mismatch happens then cpu does ``*tval =3D 3`` and raise softw= are + check exception. + +Linux kernel will treat this as :c:macro:`SIGSEV`` with code =3D +: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 / switchin= g to +shadow stack is simply writing to csr ``CSR_SSP`` changes active shadow st= ack. +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 rang= e. +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 vario= us +contexts as part of single thread. Software can be kernel as well when ker= nel +has to deliver a signal to user task and must save shadow stack pointer. K= ernel +can perform similar procedure by saving a token on user shadow stack itsel= f. +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, ke= rnel +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 shad= ow +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. --=20 2.45.0