Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

[Kees Cook <keescook@chromium.org>]

On Wed, Feb 05, 2020 at 10:19:09AM -0800, Yu-cheng Yu wrote:
> Explain no_cet_shstk/no_cet_ibt kernel parameters, and introduce a new
> document on Control-flow Enforcement Technology (CET).
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

I'm not a huge fan of the boot param names, but I can't suggest anything
better. ;) I love the extensive docs!

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  .../admin-guide/kernel-parameters.txt         |   6 +
>  Documentation/x86/index.rst                   |   1 +
>  Documentation/x86/intel_cet.rst               | 294 ++++++++++++++++++
>  3 files changed, 301 insertions(+)
>  create mode 100644 Documentation/x86/intel_cet.rst
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ade4e6ec23e0..8b69ebf0baed 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3001,6 +3001,12 @@
>  			noexec=on: enable non-executable mappings (default)
>  			noexec=off: disable non-executable mappings
>  
> +	no_cet_shstk	[X86-64] Disable Shadow Stack for user-mode
> +			applications
> +
> +	no_cet_ibt	[X86-64] Disable Indirect Branch Tracking for user-mode
> +			applications
> +
>  	nosmap		[X86,PPC]
>  			Disable SMAP (Supervisor Mode Access Prevention)
>  			even if it is supported by processor.
> diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> index a8de2fbc1caa..81f919801765 100644
> --- a/Documentation/x86/index.rst
> +++ b/Documentation/x86/index.rst
> @@ -19,6 +19,7 @@ x86-specific Documentation
>     tlb
>     mtrr
>     pat
> +   intel_cet
>     intel_mpx
>     intel-iommu
>     intel_txt
> diff --git a/Documentation/x86/intel_cet.rst b/Documentation/x86/intel_cet.rst
> new file mode 100644
> index 000000000000..71e2462fea5c
> --- /dev/null
> +++ b/Documentation/x86/intel_cet.rst
> @@ -0,0 +1,294 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=========================================
> +Control-flow Enforcement Technology (CET)
> +=========================================
> +
> +[1] Overview
> +============
> +
> +Control-flow Enforcement Technology (CET) provides protection against
> +return/jump-oriented programming (ROP) attacks.  It can be setup to
> +protect both applications and the kernel.  In the first phase, only
> +user-mode protection is implemented in the 64-bit kernel; 32-bit
> +applications are supported in compatibility mode.
> +
> +CET introduces Shadow Stack (SHSTK) and Indirect Branch Tracking
> +(IBT).  SHSTK is a secondary stack allocated from memory and cannot
> +be directly modified by applications.  When executing a CALL, the
> +processor pushes a copy of the return address to SHSTK.  Upon
> +function return, the processor pops the SHSTK copy and compares it
> +to the one from the program stack.  If the two copies differ, the
> +processor raises a control-protection fault.  IBT verifies indirect
> +CALL/JMP targets are intended as marked by the compiler with 'ENDBR'
> +opcodes (see CET instructions below).
> +
> +There are two kernel configuration options:
> +
> +    X86_INTEL_SHADOW_STACK_USER, and
> +    X86_INTEL_BRANCH_TRACKING_USER.
> +
> +To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or later
> +are required.  To build a CET-enabled application, GLIBC v2.28 or
> +later is also required.
> +
> +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
> +
> +          |----------------------|           |----------------------|
> +   addr=x |                      |           |                      |
> +          |----------------------|           |----------------------|
> +    ssp-> | rstor_token=(y|Pv|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. 'Pv' is bit[1] of a rstor_token indicating the token is to
> +           be used only for the next SAVEPREVSSP and invalid for
> +           RSTORSSP.
> +
> +SAVEPREVSSP
> +    Pop the SHSTK 'restore token' pointed by current SHSTK pointer
> +    and store it at (previous SHSTK pointer - 8).
> +
> +::
> +
> +                               After SAVEPREVSSP
> +
> +          |----------------------|           |----------------------|
> +    ssp-> |                      |           |                      |
> +          |----------------------|           |----------------------|
> + addr=x-8 | rstor_token=(y|Pv|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 and NOTRACK prefix
> +    When IBT is enabled, an indirect CALL/JMP must either::
> +
> +        have a NOTRACK prefix,
> +        reach an ENDBR, or
> +        reach an address within a legacy code page;
> +
> +    or it results in a control-protection fault.
> +
> +    When the target address is derived from information that cannot
> +    be modified, the compiler uses the NOTRACK prefix.  In other
> +    cases, the compiler inserts an ENDBR at the target address.
> +
> +    A legacy code page is designated in the legacy code bitmap, which
> +    is explained below in section [8].
> +
> +[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_X86_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_X86_CET_DISABLE, unsigned long features)
> +    Disable SHSTK and/or IBT specified in 'features'.  Return -EPERM
> +    if CET is locked.
> +
> +arch_prctl(ARCH_X86_CET_LOCK)
> +    Lock in CET feature.
> +
> +arch_prctl(ARCH_X86_CET_ALLOC_SHSTK, unsigned long *addr)
> +    Allocate a new SHSTK and put a restore token at top.
> +
> +    The parameter 'addr' is a pointer to a user buffer and indicates
> +    the desired SHSTK size to allocate.  On returning to the caller,
> +    the kernel fills '*addr' with the base address of the new SHSTK.
> +
> +arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE, unsigned long *addr)
> +    Mark an address range as IBT legacy code.
> +
> +    The parameter 'addr' is a pointer to a user buffer that has the
> +    following information::
> +
> +        *addr       = starting linear address of the legacy code
> +        *(addr + 1) = size of the legacy code
> +        *(addr + 2) = set (1); clear (0)
> +
> +Note:
> +  There is no CET-enabling arch_prctl function.  By design, CET is
> +  enabled automatically if the binary and the system can support it.
> +
> +  The parameters passed are always unsigned 64-bit.  When an IA32
> +  application passing pointers, it should only use the lower 32 bits.
> +
> +[6] The implementation of the SHSTK
> +===================================
> +
> +SHSTK size
> +----------
> +
> +A task's SHSTK is allocated from memory to a fixed size of
> +RLIMIT_STACK.  A compat-mode thread's SHSTK size is 1/4 of
> +RLIMIT_STACK.  The smaller 32-bit thread SHSTK allows more threads to
> +share a 32-bit address space.
> +
> +Signal
> +------
> +
> +The main program and its signal handlers use the same SHSTK.  Because
> +the SHSTK stores only return addresses, a large SHSTK will 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.
> +
> +IBT for signal delivering and sigreturn is the same as the main
> +program's setup; except for WAIT_ENDBR status, which can be read from
> +MSR_IA32_U_CET.  In general, a task is in WAIT_ENDBR after an
> +indirect CALL/JMP and before the next instruction starts.
> +
> +A task's WAIT_ENDBR is reset for its signal handler, but preserved on
> +the task's stack; and then restored from sigreturn.
> +
> +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_X86_CET_ALLOC_SHSTK 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 legacy bitmap
> +===========================================
> +
> +When IBT is active, a non-IBT-capable legacy library can be executed
> +if its address ranges are specified in the legacy code bitmap.  The
> +bitmap covers the whole user-space address, which is TASK_SIZE_MAX
> +for 64-bit and TASK_SIZE for IA32, and its each bit indicates a 4-KB
> +legacy code page.  It is read-only from an application, and setup by
> +the kernel as a special mapping when the first time the application
> +calls arch_prctl(ARCH_X86_CET_MARK_LEGACY_CODE).  The application
> +manages the bitmap through the arch_prctl.
> -- 
> 2.21.0
> 

-- 
Kees Cook