Date:   Sun, 30 Jan 2022 13:18:04 -0800
From:   Rick Edgecombe <rick.p.edgecombe@...el.com>
To:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com
Cc:     rick.p.edgecombe@...el.com, Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: [PATCH 01/35] Documentation/x86: Add CET description

From: Yu-cheng Yu <yu-cheng.yu@...el.com>

Introduce a new document on Control-flow Enforcement Technology (CET).

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Kees Cook <keescook@...omium.org>

---

v1:
 - Update and clarify the docs.
 - Moved kernel parameters documentation to other patch.

 Documentation/x86/cet.rst   | 145 ++++++++++++++++++++++++++++++++++++
 Documentation/x86/index.rst |   1 +
 2 files changed, 146 insertions(+)
 create mode 100644 Documentation/x86/cet.rst

diff --git a/Documentation/x86/cet.rst b/Documentation/x86/cet.rst
new file mode 100644
index 000000000000..ff0f9a148959
--- /dev/null
+++ b/Documentation/x86/cet.rst
@@ -0,0 +1,145 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================================
+Control-flow Enforcement Technology (CET)
+=========================================
+
+[1] Overview
+============
+
+Control-flow Enforcement Technology (CET) is term referring to several
+related x86 processor features that provides protection against control
+flow hijacking attacks. The HW feature itself can be set up to protect
+both applications and the kernel. Only user-mode protection is implemented
+in the 64-bit kernel, including shadow stack support for running legacy
+32-bit applications.
+
+CET introduces Shadow Stack and Indirect Branch Tracking. Shadow stack is
+a secondary stack allocated from memory and 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. Indirect branch tracking 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
+and only Shadow Stack is currently supported in the kernel.
+
+The Kconfig options is X86_SHADOW_STACK, and it can be disabled with
+no_user_shstk.
+
+To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or LLVM v10.0.1
+or later are required. To build a CET-enabled application, GLIBC v2.28 or
+later is also required.
+
+At run time, /proc/cpuinfo shows CET features if the processor supports
+CET.
+
+[2] Application Enabling
+========================
+
+An application's CET capability is marked in its ELF header and can be
+verified from readelf/llvm-readelf output:
+
+    readelf -n <application> | grep -a SHSTK
+        properties: x86 feature: SHSTK
+
+The kernel does not process these applications directly. Applications must
+enable them using the interface descriped in section 4. Typically this
+would be done in dynamic loader or static runtime objects, as is the case
+in glibc.
+
+[3] Backward Compatibility
+==========================
+
+GLIBC provides a few CET tunables via the GLIBC_TUNABLES environment
+variable:
+
+GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK,-WRSS
+    Turn off SHSTK/WRSS.
+
+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.
+
+Details can be found in the GLIBC manual pages.
+
+[4] CET arch_prctl()'s
+======================
+
+Elf features are enabled using the below arch_prctl's.
+
+arch_prctl(ARCH_X86_FEATURE_STATUS, u64 *args)
+    Get feature status.
+
+    The parameter 'args' is a pointer to a user buffer. The kernel returns
+    the following information:
+
+    *args = shadow stack/IBT status
+    *(args + 1) = shadow stack base address
+    *(args + 2) = shadow stack size
+
+    32-bit binaries use the same interface, but only lower 32-bits of each
+    item.
+
+arch_prctl(ARCH_X86_FEATURE_DISABLE, unsigned int features)
+    Disable features specified in 'features'. Return -EPERM if any of the
+    passed feature are locked. Return -ECANCELED if any of the features
+    failed to disable. In this case call ARCH_X86_FEATURE_STATUS to find
+    out which features are still enabled.
+
+arch_prctl(ARCH_X86_FEATURE_ENABLE, unsigned int features)
+    Enable feature specified in 'features'. Return -EPERM if any of the
+    passed feature are locked. Return -ECANCELED if any of the features
+    failed to enable. In this case call ARCH_X86_FEATURE_STATUS to find
+    out which features were enabled.
+
+arch_prctl(ARCH_X86_FEATURE_LOCK, unsigned int features)
+    Lock in all features at their current enabled or disabled status.
+
+
+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.
+
+[5] The 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. However,
+a compat-mode application's address space is smaller, each of its thread's
+shadow stack size is MIN(1/4 RLIMIT_STACK, 4 GB).
+
+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.
+
+The kernel creates a restore token for the shadow stack and pushes the
+restorer address to the shadow stack. Then verifies that token when
+restoring from the signal handler.
+
+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.
diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index f498f1d36cd3..b5f083a61eab 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -21,6 +21,7 @@ x86-specific Documentation
    tlb
    mtrr
    pat
+   cet
    intel-iommu
    intel_txt
    amd-memory-encryption
-- 
2.17.1

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