Merge tag 'x86_shstk_for_6.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 shadow stack support from Dave Hansen:
 "This is the long awaited x86 shadow stack support, part of Intel's
  Control-flow Enforcement Technology (CET).

  CET consists of two related security features: shadow stacks and
  indirect branch tracking. This series implements just the shadow stack
  part of this feature, and just for userspace.

  The main use case for shadow stack is providing protection against
  return oriented programming attacks. It works by maintaining a
  secondary (shadow) stack using a special memory type that has
  protections against modification. When executing a CALL instruction,
  the processor pushes the return address to both the normal stack and
  to the special permission shadow stack. Upon RET, the processor pops
  the shadow stack copy and compares it to the normal stack copy.

  For more information, refer to the links below for the earlier
  versions of this patch set"

Link: https://lore.kernel.org/lkml/20220130211838.8382-1-rick.p.edgecombe@intel.com/
Link: https://lore.kernel.org/lkml/20230613001108.3040476-1-rick.p.edgecombe@intel.com/

* tag 'x86_shstk_for_6.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (47 commits)
  x86/shstk: Change order of __user in type
  x86/ibt: Convert IBT selftest to asm
  x86/shstk: Don't retry vm_munmap() on -EINTR
  x86/kbuild: Fix Documentation/ reference
  x86/shstk: Move arch detail comment out of core mm
  x86/shstk: Add ARCH_SHSTK_STATUS
  x86/shstk: Add ARCH_SHSTK_UNLOCK
  x86: Add PTRACE interface for shadow stack
  selftests/x86: Add shadow stack test
  x86/cpufeatures: Enable CET CR4 bit for shadow stack
  x86/shstk: Wire in shadow stack interface
  x86: Expose thread features in /proc/$PID/status
  x86/shstk: Support WRSS for userspace
  x86/shstk: Introduce map_shadow_stack syscall
  x86/shstk: Check that signal frame is shadow stack mem
  x86/shstk: Check that SSP is aligned on sigreturn
  x86/shstk: Handle signals for shadow stack
  x86/shstk: Introduce routines modifying shstk
  x86/shstk: Handle thread shadow stack
  x86/shstk: Add user-mode shadow stack support
  ...

2 files changed, 180 insertions, 0 deletions

diff --git a/Documentation/arch/x86/index.rst b/Documentation/arch/x86/index.rst
index c73d133fd37c78..8ac64d7de4dc9a 100644
--- a/Documentation/arch/x86/index.rst
+++ b/Documentation/arch/x86/index.rst
@@ -22,6 +22,7 @@ x86-specific Documentation
    mtrr
    pat
    intel-hfi
+   shstk
    iommu
    intel_txt
    amd-memory-encryption
diff --git a/Documentation/arch/x86/shstk.rst b/Documentation/arch/x86/shstk.rst
new file mode 100644
index 00000000000000..60260e809baf6a
--- /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 <application> | 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.