From ace9429bb58fd418f0c81d4c2835699bddf6bde6 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Thu, 11 Apr 2024 10:27:49 +0200 Subject: Adding upstream version 6.6.15. Signed-off-by: Daniel Baumann --- Documentation/arch/x86/shstk.rst | 179 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 179 insertions(+) create mode 100644 Documentation/arch/x86/shstk.rst (limited to 'Documentation/arch/x86/shstk.rst') diff --git a/Documentation/arch/x86/shstk.rst b/Documentation/arch/x86/shstk.rst new file mode 100644 index 0000000000..60260e809b --- /dev/null +++ b/Documentation/arch/x86/shstk.rst @@ -0,0 +1,179 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================================================== +Control-flow Enforcement Technology (CET) Shadow Stack +====================================================== + +CET Background +============== + +Control-flow Enforcement Technology (CET) covers several related x86 processor +features that provide protection against control flow hijacking attacks. CET +can protect both applications and the kernel. + +CET introduces shadow stack and indirect branch tracking (IBT). A shadow stack +is a secondary stack allocated from memory which cannot be directly modified by +applications. When executing a CALL instruction, the processor pushes the +return address to both the normal stack and the shadow stack. Upon +function return, the processor pops the shadow stack copy and compares it +to the normal stack copy. If the two differ, the processor raises a +control-protection fault. IBT verifies indirect CALL/JMP targets are intended +as marked by the compiler with 'ENDBR' opcodes. Not all CPU's have both Shadow +Stack and Indirect Branch Tracking. Today in the 64-bit kernel, only userspace +shadow stack and kernel IBT are supported. + +Requirements to use Shadow Stack +================================ + +To use userspace shadow stack you need HW that supports it, a kernel +configured with it and userspace libraries compiled with it. + +The kernel Kconfig option is X86_USER_SHADOW_STACK. When compiled in, shadow +stacks can be disabled at runtime with the kernel parameter: nousershstk. + +To build a user shadow stack enabled kernel, Binutils v2.29 or LLVM v6 or later +are required. + +At run time, /proc/cpuinfo shows CET features if the processor supports +CET. "user_shstk" means that userspace shadow stack is supported on the current +kernel and HW. + +Application Enabling +==================== + +An application's CET capability is marked in its ELF note and can be verified +from readelf/llvm-readelf output:: + + readelf -n | grep -a SHSTK + properties: x86 feature: SHSTK + +The kernel does not process these applications markers directly. Applications +or loaders must enable CET features using the interface described in section 4. +Typically this would be done in dynamic loader or static runtime objects, as is +the case in GLIBC. + +Enabling arch_prctl()'s +======================= + +Elf features should be enabled by the loader using the below arch_prctl's. They +are only supported in 64 bit user applications. These operate on the features +on a per-thread basis. The enablement status is inherited on clone, so if the +feature is enabled on the first thread, it will propagate to all the thread's +in an app. + +arch_prctl(ARCH_SHSTK_ENABLE, unsigned long feature) + Enable a single feature specified in 'feature'. Can only operate on + one feature at a time. + +arch_prctl(ARCH_SHSTK_DISABLE, unsigned long feature) + Disable a single feature specified in 'feature'. Can only operate on + one feature at a time. + +arch_prctl(ARCH_SHSTK_LOCK, unsigned long features) + Lock in features at their current enabled or disabled status. 'features' + is a mask of all features to lock. All bits set are processed, unset bits + are ignored. The mask is ORed with the existing value. So any feature bits + set here cannot be enabled or disabled afterwards. + +arch_prctl(ARCH_SHSTK_UNLOCK, unsigned long features) + Unlock features. 'features' is a mask of all features to unlock. All + bits set are processed, unset bits are ignored. Only works via ptrace. + +arch_prctl(ARCH_SHSTK_STATUS, unsigned long addr) + Copy the currently enabled features to the address passed in addr. The + features are described using the bits passed into the others in + 'features'. + +The return values are as follows. On success, return 0. On error, errno can +be:: + + -EPERM if any of the passed feature are locked. + -ENOTSUPP if the feature is not supported by the hardware or + kernel. + -EINVAL arguments (non existing feature, etc) + -EFAULT if could not copy information back to userspace + +The feature's bits supported are:: + + ARCH_SHSTK_SHSTK - Shadow stack + ARCH_SHSTK_WRSS - WRSS + +Currently shadow stack and WRSS are supported via this interface. WRSS +can only be enabled with shadow stack, and is automatically disabled +if shadow stack is disabled. + +Proc Status +=========== +To check if an application is actually running with shadow stack, the +user can read the /proc/$PID/status. It will report "wrss" or "shstk" +depending on what is enabled. The lines look like this:: + + x86_Thread_features: shstk wrss + x86_Thread_features_locked: shstk wrss + +Implementation of the Shadow Stack +================================== + +Shadow Stack Size +----------------- + +A task's shadow stack is allocated from memory to a fixed size of +MIN(RLIMIT_STACK, 4 GB). In other words, the shadow stack is allocated to +the maximum size of the normal stack, but capped to 4 GB. In the case +of the clone3 syscall, there is a stack size passed in and shadow stack +uses this instead of the rlimit. + +Signal +------ + +The main program and its signal handlers use the same shadow stack. Because +the shadow stack stores only return addresses, a large shadow stack covers +the condition that both the program stack and the signal alternate stack run +out. + +When a signal happens, the old pre-signal state is pushed on the stack. When +shadow stack is enabled, the shadow stack specific state is pushed onto the +shadow stack. Today this is only the old SSP (shadow stack pointer), pushed +in a special format with bit 63 set. On sigreturn this old SSP token is +verified and restored by the kernel. The kernel will also push the normal +restorer address to the shadow stack to help userspace avoid a shadow stack +violation on the sigreturn path that goes through the restorer. + +So the shadow stack signal frame format is as follows:: + + |1...old SSP| - Pointer to old pre-signal ssp in sigframe token format + (bit 63 set to 1) + | ...| - Other state may be added in the future + + +32 bit ABI signals are not supported in shadow stack processes. Linux prevents +32 bit execution while shadow stack is enabled by the allocating shadow stacks +outside of the 32 bit address space. When execution enters 32 bit mode, either +via far call or returning to userspace, a #GP is generated by the hardware +which, will be delivered to the process as a segfault. When transitioning to +userspace the register's state will be as if the userspace ip being returned to +caused the segfault. + +Fork +---- + +The shadow stack's vma has VM_SHADOW_STACK flag set; its PTEs are required +to be read-only and dirty. When a shadow stack PTE is not RO and dirty, a +shadow access triggers a page fault with the shadow stack access bit set +in the page fault error code. + +When a task forks a child, its shadow stack PTEs are copied and both the +parent's and the child's shadow stack PTEs are cleared of the dirty bit. +Upon the next shadow stack access, the resulting shadow stack page fault +is handled by page copy/re-use. + +When a pthread child is created, the kernel allocates a new shadow stack +for the new thread. New shadow stack creation behaves like mmap() with respect +to ASLR behavior. Similarly, on thread exit the thread's shadow stack is +disabled. + +Exec +---- + +On exec, shadow stack features are disabled by the kernel. At which point, +userspace can choose to re-enable, or lock them. -- cgit v1.2.3