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[192.55.52.136]) by mx.google.com with ESMTPS id 91si31377plh.398.2019.06.06.13.15.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 06 Jun 2019 13:15:10 -0700 (PDT) Received-SPF: pass (google.com: domain of yu-cheng.yu@intel.com designates 192.55.52.136 as permitted sender) client-ip=192.55.52.136; Authentication-Results: mx.google.com; spf=pass (google.com: domain of yu-cheng.yu@intel.com designates 192.55.52.136 as permitted sender) smtp.mailfrom=yu-cheng.yu@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 06 Jun 2019 13:15:09 -0700 X-ExtLoop1: 1 Received: from yyu32-desk1.sc.intel.com ([143.183.136.147]) by orsmga002.jf.intel.com with ESMTP; 06 Jun 2019 13:15:08 -0700 From: Yu-cheng Yu To: x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin Cc: Yu-cheng Yu Subject: [PATCH v7 01/27] Documentation/x86: Add CET description Date: Thu, 6 Jun 2019 13:06:20 -0700 Message-Id: <20190606200646.3951-2-yu-cheng.yu@intel.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20190606200646.3951-1-yu-cheng.yu@intel.com> References: <20190606200646.3951-1-yu-cheng.yu@intel.com> 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: X-Virus-Scanned: ClamAV using ClamSMTP Explain how CET works and the no_cet_shstk/no_cet_ibt kernel parameters. Signed-off-by: Yu-cheng Yu --- .../admin-guide/kernel-parameters.txt | 6 + Documentation/x86/index.rst | 1 + Documentation/x86/intel_cet.rst | 268 ++++++++++++++++++ 3 files changed, 275 insertions(+) create mode 100644 Documentation/x86/intel_cet.rst diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 138f6664b2e2..b9b054f6c732 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2906,6 +2906,12 @@ noexec=on: enable non-executable mappings (default) noexec=off: disable non-executable mappings + no_cet_ibt [X86-64] Disable indirect branch tracking for user-mode + applications + + no_cet_shstk [X86-64] Disable shadow stack support for user-mode + applications + nosmap [X86,PPC] Disable SMAP (Supervisor Mode Access Prevention) even if it is supported by processor. diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index ae36fc5fc649..47822ac02fe0 100644 --- a/Documentation/x86/index.rst +++ b/Documentation/x86/index.rst @@ -21,6 +21,7 @@ x86-specific Documentation pat protection-keys intel_mpx + intel_cet amd-memory-encryption pti mds diff --git a/Documentation/x86/intel_cet.rst b/Documentation/x86/intel_cet.rst new file mode 100644 index 000000000000..dac83bbf8a24 --- /dev/null +++ b/Documentation/x86/intel_cet.rst @@ -0,0 +1,268 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +Control-flow Enforcement Technology (CET) +========================================= + +[1] Overview +============ + +Control-flow Enforcement Technology (CET) provides protection against +return/jump-oriented programming (ROP) attacks. It can be setup to +protect both the kernel and applications. In the first phase, +only the user-mode protection is implemented in 64-bit mode; 32-bit +applications are supported in compatibility mode. + +CET introduces shadow stack (SHSTK) and indirect branch tracking +(IBT). SHSTK is a secondary stack allocated from memory and cannot +be directly modified by applications. When executing a CALL, the +processor pushes a copy of the return address to SHSTK. Upon +function return, the processor pops the SHSTK copy and compares it +to the one from the program stack. If the two copies differ, the +processor raises a control-protection exception. IBT verifies all +indirect CALL/JMP targets are intended as marked by the compiler +with 'ENDBR' opcodes (see CET instructions below). + +There are two kernel configuration options: + + INTEL_X86_SHADOW_STACK_USER, and + INTEL_X86_BRANCH_TRACKING_USER. + +To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or later +are required. To build a CET-enabled application, GLIBC v2.28 or +later is also required. + +There are two command-line options for disabling CET features: + + no_cet_shstk - disables SHSTK, and + no_cet_ibt - disables IBT. + +At run time, /proc/cpuinfo shows the availability of SHSTK and IBT. + +[2] CET assembly instructions +============================= + +RDSSP %r + Read the SHSTK pointer into %r. + +INCSSP %r + Unwind (increment) the SHSTK pointer (0 ~ 255) steps as indicated + in the operand register. The GLIBC longjmp uses INCSSP to unwind + the SHSTK until that matches the program stack. When it is + necessary to unwind beyond 255 steps, longjmp divides and repeats + the process. + +RSTORSSP (%r) + Switch to the SHSTK indicated in the 'restore token' pointed by + the operand register and replace the 'restore token' with a new + token to be saved (with SAVEPREVSSP) for the outgoing SHSTK. + +:: + + Before RSTORSSP + + Incoming SHSTK Current/Outgoing SHSTK + + |----------------------| |----------------------| + addr=x | | ssp-> | | + |----------------------| |----------------------| + (%r)-> | rstor_token=(x|Lg) | addr=y-8 | | + |----------------------| |----------------------| + + After RSTORSSP + + |----------------------| |----------------------| + | | | | + |----------------------| |----------------------| + ssp-> | rstor_token=(y|Bz|Lg)| addr=y-8 | | + |----------------------| |----------------------| + + note: + 1. Only valid addresses and restore tokens can be on the + user-mode SHSTK. + 2. A token is always of type u64 and must align to u64. + 3. The incoming SHSTK pointer in a rstor_token must point to + immediately above the token. + 4. 'Lg' is bit[0] of a rstor_token indicating a 64-bit SHSTK. + 5. 'Bz' is bit[1] of a rstor_token indicating the token is to + be used only for the next SAVEPREVSSP and invalid for the + RSTORSSP. + +SAVEPREVSSP + Store the SHSTK 'restore token' pointed by + (current_SHSTK_pointer + 8). + +:: + + After SAVEPREVSSP + + |----------------------| |----------------------| + ssp-> | | | | + |----------------------| |----------------------| + | rstor_token=(y|Bz|Lg)| addr=y-8 | rstor_token(y|Lg) | + |----------------------| |----------------------| + +WRUSS %r0, (%r1) + Write the value in %r0 to the SHSTK address pointed by (%r1). + This is a kernel-mode only instruction. + +ENDBR + The compiler inserts an ENDBR at all valid branch targets. Any + CALL/JMP to a target without an ENDBR triggers a control + protection fault. + +[3] Application Enabling +======================== + +An application's CET capability is marked in its ELF header and can +be verified from the following command output, in the +NT_GNU_PROPERTY_TYPE_0 field: + + readelf -n + +If an application supports CET and is statically linked, it will run +with CET protection. If the application needs any shared libraries, +the loader checks all dependencies and enables CET only when all +requirements are met. + +[4] Legacy Libraries +==================== + +GLIBC provides a few tunables for backward compatibility. + +GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK,-IBT + Turn off SHSTK/IBT for the current shell. + +GLIBC_TUNABLES=glibc.tune.x86_shstk= + This controls how dlopen() handles SHSTK legacy libraries: + on: continue with SHSTK enabled; + permissive: continue with SHSTK off. + +[5] CET system calls +==================== + +The following arch_prctl() system calls are added for CET: + +arch_prctl(ARCH_X86_CET_STATUS, unsigned long *addr) + Return CET feature status. + + The parameter 'addr' is a pointer to a user buffer. + On returning to the caller, the kernel fills the following + information: + + *addr = SHSTK/IBT status + *(addr + 1) = SHSTK base address + *(addr + 2) = SHSTK size + +arch_prctl(ARCH_X86_CET_DISABLE, unsigned long features) + Disable SHSTK and/or IBT specified in 'features'. Return -EPERM + if CET is locked. + +arch_prctl(ARCH_X86_CET_LOCK) + Lock in CET feature. + +arch_prctl(ARCH_X86_CET_ALLOC_SHSTK, unsigned long *addr) + Allocate a new SHSTK and put a restore token at top. + + The parameter 'addr' is a pointer to a user buffer and indicates + the desired SHSTK size to allocate. On returning to the caller, + the kernel fills *addr with the base address of the new SHSTK. + +arch_prctl(ARCH_X86_CET_SET_LEGACY_BITMAP, unsigned long *addr) + Setup an IBT legacy code bitmap. + + The parameter 'addr' is a pointer to a user buffer that has the + following information: + + *addr = IBT bitmap base address + *(addr + 1) = IBT bitmap size + +Note: + There is no CET enabling arch_prctl function. By design, CET is + enabled automatically if the binary and the system can support it. + + The parameters passed are always unsigned 64-bit. When an ia32 + application passing pointers, it should only use the lower 32 bits. + +[6] The implementation of the SHSTK +=================================== + +SHSTK size +---------- + +A task's SHSTK is allocated from memory to a fixed size of +RLIMIT_STACK. A compat-mode thread's SHSTK size is 1/4 of +RLIMIT_STACK. The smaller 32-bit thread SHSTK allows more threads to +share a 32-bit address space. + +Signal +------ + +The main program and its signal handlers use the same SHSTK. Because +the SHSTK stores only return addresses, a large SHSTK will cover the +condition that both the program stack and the sigaltstack run out. + +The kernel creates a restore token at the SHSTK restoring address and +verifies that token when restoring from the signal handler. + +Fork +---- + +The SHSTK's vma has VM_SHSTK flag set; its PTEs are required to be +read-only and dirty. When a SHSTK PTE is not present, RO, and dirty, +a SHSTK access triggers a page fault with an additional SHSTK bit set +in the page fault error code. + +When a task forks a child, its SHSTK PTEs are copied and both the +parent's and the child's SHSTK PTEs are cleared of the dirty bit. +Upon the next SHSTK access, the resulting SHSTK page fault is handled +by page copy/re-use. + +When a pthread child is created, the kernel allocates a new SHSTK for +the new thread. + +Setjmp/Longjmp +-------------- + +Longjmp unwinds SHSTK until it matches the program stack. + +Ucontext +-------- + +In GLIBC, getcontext/setcontext is implemented in similar way as +setjmp/longjmp. + +When makecontext creates a new ucontext, a new SHSTK is allocated for +that context with ARCH_X86_CET_ALLOC_SHSTK the syscall. The kernel +creates a restore token at the top of the new SHSTK and the user-mode +code switches to the new SHSTK with the RSTORSSP instruction. + +[7] The management of read-only & dirty PTEs for SHSTK +====================================================== + +A RO and dirty PTE exists in the following cases: + +(a) A page is modified and then shared with a fork()'ed child; +(b) A R/O page that has been COW'ed; +(c) A SHSTK page. + +The processor only checks the dirty bit for (c). To prevent the use +of non-SHSTK memory as SHSTK, we use a spare bit of the 64-bit PTE as +DIRTY_SW for (a) and (b) above. This results to the following PTE +settings: + +Modified PTE: (R/W + DIRTY_HW) +Modified and shared PTE: (R/O + DIRTY_SW) +R/O PTE, COW'ed: (R/O + DIRTY_SW) +SHSTK PTE: (R/O + DIRTY_HW) +SHSTK PTE, COW'ed: (R/O + DIRTY_HW) +SHSTK PTE, shared: (R/O + DIRTY_SW) + +Note that DIRTY_SW is only used in R/O PTEs but not R/W PTEs. + +[8] The implementation of IBT +============================= + +The kernel provides IBT support in mmap() of the legacy code bit map. +However, the management of the bitmap is done in the GLIBC or the +application.