Commit df57721f authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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
  ...
parents b97d64c7 1fe428d3

Documentation/arch/x86/index.rst

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@@ -22,6 +22,7 @@ x86-specific Documentation 
   mtrr

   pat

   intel-hfi

   shstk

   iommu

   intel_txt

   amd-memory-encryption

Documentation/arch/x86/shstk.rst

0 → 100644
+179 −0
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.. 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.

Documentation/filesystems/proc.rst

+1 −0
Original line number Diff line number Diff line
@@ -566,6 +566,7 @@ encoded manner. The codes are the following: 
    mt    arm64 MTE allocation tags are enabled

    um    userfaultfd missing tracking

    uw    userfaultfd wr-protect tracking

    ss    shadow stack page

    ==    =======================================



Note that there is no guarantee that every flag and associated mnemonic will

Documentation/mm/arch_pgtable_helpers.rst

+10 −2
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@@ -46,7 +46,11 @@ PTE Page Table Helpers 
+---------------------------+--------------------------------------------------+

| pte_mkclean               | Creates a clean PTE                              |

+---------------------------+--------------------------------------------------+

| pte_mkwrite               | Creates a writable PTE                           |

| pte_mkwrite               | Creates a writable PTE of the type specified by  |

|                           | the VMA.                                         |

+---------------------------+--------------------------------------------------+

| pte_mkwrite_novma         | Creates a writable PTE, of the conventional type |

|                           | of writable.                                     |

+---------------------------+--------------------------------------------------+

| pte_wrprotect             | Creates a write protected PTE                    |

+---------------------------+--------------------------------------------------+
@@ -118,7 +122,11 @@ PMD Page Table Helpers 
+---------------------------+--------------------------------------------------+

| pmd_mkclean               | Creates a clean PMD                              |

+---------------------------+--------------------------------------------------+

| pmd_mkwrite               | Creates a writable PMD                           |

| pmd_mkwrite               | Creates a writable PMD of the type specified by  |

|                           | the VMA.                                         |

+---------------------------+--------------------------------------------------+

| pmd_mkwrite_novma         | Creates a writable PMD, of the conventional type |

|                           | of writable.                                     |

+---------------------------+--------------------------------------------------+

| pmd_wrprotect             | Creates a write protected PMD                    |

+---------------------------+--------------------------------------------------+

arch/Kconfig

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@@ -931,6 +931,14 @@ config HAVE_ARCH_HUGE_VMALLOC 
config ARCH_WANT_HUGE_PMD_SHARE

	bool



# Archs that want to use pmd_mkwrite on kernel memory need it defined even

# if there are no userspace memory management features that use it

config ARCH_WANT_KERNEL_PMD_MKWRITE

	bool



config ARCH_WANT_PMD_MKWRITE

	def_bool TRANSPARENT_HUGEPAGE || ARCH_WANT_KERNEL_PMD_MKWRITE



config HAVE_ARCH_SOFT_DIRTY

	bool