> index ade4e6ec23e0..8b69ebf0baed 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -3001,6 +3001,12 @@ > noexec=on: enable non-executable mappings (default) > noexec=off: disable non-executable mappings > > + no_cet_shstk [X86-64] Disable Shadow Stack for user-mode > + applications If we ever add kernel support, "no_cet_shstk" will mean "no cet shstk for userspace"? > + no_cet_ibt [X86-64] Disable Indirect Branch Tracking for user-mode > + applications > + > nosmap [X86,PPC] > Disable SMAP (Supervisor Mode Access Prevention) > even if it is supported by processor. BTW, this documentation is misplaced. It needs to go to the spot where you introduce the code for these options. > diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst > index a8de2fbc1caa..81f919801765 100644 > --- a/Documentation/x86/index.rst > +++ b/Documentation/x86/index.rst > @@ -19,6 +19,7 @@ x86-specific Documentation > tlb > mtrr > pat > + intel_cet > intel_mpx > intel-iommu > intel_txt > diff --git a/Documentation/x86/intel_cet.rst b/Documentation/x86/intel_cet.rst > new file mode 100644 > index 000000000000..71e2462fea5c > --- /dev/null > +++ b/Documentation/x86/intel_cet.rst > @@ -0,0 +1,294 @@ > +.. 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 ^ set up > +protect both applications and the kernel. In the first phase, only > +user-mode protection is implemented in the 64-bit kernel; 32-bit > +applications are supported in compatibility mode. Please just say what *is* at the time of the writing. We don't need to talk about "phases". Also, you haven't mentioned that this is a *hardware* feature and that it's only on Intel CPUs at the moment. That's kinda essential. If I've got an AMD CPU, I can just stop reading. :) The hardware supports shadow stacks for both userspace and the kernel in both 32 and 64-bit modes. 32-bit kernel support is not implemented. Both 32-bit and 64-bit user applications can run on 64-bit kernels. This is also missing the same key points about enabling as the Kconfig text: apps don't get this for free and must be specifically enabled. > +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. ... and to the normal stack > 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 fault. IBT verifies indirect > +CALL/JMP targets are intended as marked by the compiler with 'ENDBR' > +opcodes (see CET instructions below). > + > +There are two kernel configuration options: > + > + X86_INTEL_SHADOW_STACK_USER, and > + X86_INTEL_BRANCH_TRACKING_USER. > + > +To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or later > +are required. Why are these needed to build a CET-enabled kernel? > 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. Availability of what? If I set X86_INTEL_SHADOW_STACK_USER=n, I'll still see the cpuinfo flags, but I won't have runtime support. Probably best to say that cpuinfo tells you about processor support only. > +[2] CET assembly instructions > +============================= Why do we need this in the kernel? What is specific to Linux or the kernel? Why wouldn't I just go read the SDM if I want to know how the instructions work? > +[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 <application> > + > +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. What about shared libraries loaded after the program starts? > +[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=<on, permissive> > + This controls how dlopen() handles SHSTK legacy libraries:: > + > + on - continue with SHSTK enabled; > + permissive - continue with SHSTK off. This seems like manpage fodder more than kernel documentation to me. > +[5] CET system calls > +==================== > + > +The following arch_prctl() system calls are added for CET: FWIW, I wouldn't call each of these a "system call". "Several arch_prctl()'s have been 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. Shouldn't this say what "locking" means? > +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_MARK_LEGACY_CODE, unsigned long *addr) > + Mark an address range as IBT legacy code. > + > + The parameter 'addr' is a pointer to a user buffer that has the > + following information:: > + > + *addr = starting linear address of the legacy code > + *(addr + 1) = size of the legacy code > + *(addr + 2) = set (1); clear (0) > + > +Note: > + There is no CET-enabling arch_prctl function. By design, CET is > + enabled automatically if the binary and the system can support it. This is kinda interesting. It means that a JIT couldn't choose to protect the code it generates and have different rules from itself? > + The parameters passed are always unsigned 64-bit. When an IA32 > + application passing pointers, it should only use the lower 32 bits. Won't a 32-bit app calling prctl() use the 32-bit ABI? How would it even know it's running on a 64-bit kernel? > +[6] The implementation of the SHSTK > +=================================== > + > +SHSTK size > +---------- > + > +A task's SHSTK is allocated from memory to a fixed size of > +RLIMIT_STACK. I can't really parse that sentence. Is this saying that shadow stacks are limited by and share space with normal stacks via 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. I thought the size was passed in from userspace? Where does this sizing take place? Is this a convention or is it being enforced? > +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. ^ typo? I'm not sure what this is trying to say. > +The kernel creates a restore token at the SHSTK restoring address and > +verifies that token when restoring from the signal handler. I think there's a sentence or two of background missing here. I'm really lost as to what this is trying to tell me. > +IBT for signal delivering and sigreturn is the same as the main > +program's setup; except for WAIT_ENDBR status, which can be read from > +MSR_IA32_U_CET. In general, a task is in WAIT_ENDBR after an > +indirect CALL/JMP and before the next instruction starts. I'm 100% lost. I have no idea what this is trying to tell me or why it is relevant to the kernel. > +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. What's the most important thing about shadow stacks and fork()? Does this documentation tell that to the reader? > +When a pthread child is created, the kernel allocates a new SHSTK for > +the new thread. Why is this here? Are pthread children created work fork()? > +Setjmp/Longjmp > +-------------- > + > +Longjmp unwinds SHSTK until it matches the program stack. I'm missing what this has to do with the kernel. > +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 syscall. The kernel Nit: ARCH_X86_CET_ALLOC_SHSTK is not a syscall. > +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. This seems like a howto for doing user-level threading. It seems like it could be replaced by a single sentence in the ARCH_X86_CET_ALLOC_SHSTK documentation explaining that new shadow stacks are generally (always??) allocated along with new stacks. Since new clone() threads need a new stack, they also need a new shadow stack. User-level threads that need a new stack are also expected to allocate a new shadow stack. Right? > +[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. I really don't think this belongs in the documentation, especially since it's duplicated almost verbatim in code comments. > +[8] The implementation of IBT legacy bitmap > +=========================================== > + > +When IBT is active, a non-IBT-capable legacy library can be executed > +if its address ranges are specified in the legacy code bitmap. The > +bitmap covers the whole user-space address, which is TASK_SIZE_MAX > +for 64-bit and TASK_SIZE for IA32, and its each bit indicates a 4-KB > +legacy code page. It is read-only from an application, and setup by ^ set up > +the kernel as a special mapping when the first time the application > +calls arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE). The application > +manages the bitmap through the arch_prctl.
