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Subject[RFC PATCH v2 05/27] Documentation/x86: Add CET description
Date
Explain how CET works and the no_cet_shstk/no_cet_ibt kernel
parameters.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
.../admin-guide/kernel-parameters.txt | 6 +
Documentation/x86/intel_cet.txt | 250 ++++++++++++++++++
2 files changed, 256 insertions(+)
create mode 100644 Documentation/x86/intel_cet.txt

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index efc7aa7a0670..dc787facdcde 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2661,6 +2661,12 @@
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]
Disable SMAP (Supervisor Mode Access Prevention)
even if it is supported by processor.
diff --git a/Documentation/x86/intel_cet.txt b/Documentation/x86/intel_cet.txt
new file mode 100644
index 000000000000..974bb8262146
--- /dev/null
+++ b/Documentation/x86/intel_cet.txt
@@ -0,0 +1,250 @@
+=========================================
+Control Flow Enforcement Technology (CET)
+=========================================
+
+[1] Overview
+============
+
+Control Flow Enforcement Technology (CET) provides protection against
+return/jump-oriented programing (ROP) attacks. It can be implemented
+to protect both the kernel and applications. In the first phase,
+only the user-mode protection is implemented for the 64-bit kernel.
+Thirty-two bit applications are supported under the compatibility
+mode.
+
+CET includes shadow stack (SHSTK) and indirect branch tracking (IBT)
+and they are enabled from two kernel configuration options:
+
+ INTEL_X86_SHADOW_STACK_USER, and
+ INTEL_X86_BRANCH_TRACKING_USER.
+
+To build a CET-enabled kernel, Binutils v2.30 and GCC v8.1 or later
+are required. To build a CET-enabled application, GLIBC v2.29 or
+later is also requried.
+
+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 <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.
+
+[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.
+
+[5] CET system calls
+====================
+
+The following arch_prctl() system calls are added for CET:
+
+arch_prctl(ARCH_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_CET_DISABLE, unsigned long features)
+ Disable SHSTK and/or IBT specified in 'features'. Return -EPERM
+ if CET is locked out.
+
+arch_prctl(ARCH_CET_LOCK)
+ Lock out CET feature.
+
+arch_prctl(ARCH_CET_ALLOC_SHSTK, unsigned long *addr)
+ Allocate a new SHSTK.
+
+ The parameter 'addr' is a pointer to a user buffer and indicates
+ the desired SHSTK size to allocate. On returning to the caller
+ the buffer contains the address of the new SHSTK.
+
+arch_prctl(ARCH_CET_LEGACY_BITMAP, unsigned long *addr)
+ Allocate an IBT legacy code bitmap if the current task does not
+ have one.
+
+ The parameter 'addr' is a pointer to a user buffer.
+ On returning to the caller, the kernel fills the following
+ information:
+
+ *addr = IBT bitmap base address
+ *(addr + 1) = IBT bitmap size
+
+[6] The implementation of the SHSTK
+===================================
+
+SHSTK size
+----------
+
+A task's SHSTK is allocated from memory to a fixed size that can
+support 32 KB nested function calls; that is 256 KB for a 64-bit
+application and 128 KB for a 32-bit application. The system admin
+can change the default size.
+
+Signal
+------
+
+The main program and its signal handlers use the same SHSTK. Because
+the SHSTK stores only return addresses, we can estimate a large
+enough SHSTK to 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_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.
--
2.17.1
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