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

[Dave Hansen]

> 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

If we ever add kernel support, "no_cet_shstk" will mean "no cet shstk
for userspace"?

> +	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.

BTW, this documentation is misplaced.  It needs to go to the spot where
you introduce the code for these options.

> 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

							      ^ set up

> +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.

Please just say what *is* at the time of the writing.  We don't need to
talk about "phases".

Also, you haven't mentioned that this is a *hardware* feature and that
it's only on Intel CPUs at the moment.  That's kinda essential.  If I've
got an AMD CPU, I can just stop reading. :)

The hardware supports shadow stacks for both userspace and the kernel in
both 32 and 64-bit modes.  32-bit kernel support is not implemented.
Both 32-bit and 64-bit user applications can run on 64-bit kernels.

This is also missing the same key points about enabling as the Kconfig
text: apps don't get this for free and must be specifically enabled.

> +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. 

... and to the normal stack

>  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.

Why are these needed to build a CET-enabled kernel?

>  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.

Availability of what?

If I set X86_INTEL_SHADOW_STACK_USER=n, I'll still see the cpuinfo
flags, but I won't have runtime support.

Probably best to say that cpuinfo tells you about processor support only.

> +[2] CET assembly instructions
> +=============================

Why do we need this in the kernel?  What is specific to Linux or the
kernel?  Why wouldn't I just go read the SDM if I want to know how the
instructions work?

> +[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.

What about shared libraries loaded after the program starts?

> +[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.

This seems like manpage fodder more than kernel documentation to me.

> +[5] CET system calls
> +====================
> +
> +The following arch_prctl() system calls are added for CET:

FWIW, I wouldn't call each of these a "system call".

"Several arch_prctl()'s have been 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.

Shouldn't this say what "locking" means?

> +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.

This is kinda interesting.  It means that a JIT couldn't choose to
protect the code it generates and have different rules from itself?

> +  The parameters passed are always unsigned 64-bit.  When an IA32
> +  application passing pointers, it should only use the lower 32 bits.

Won't a 32-bit app calling prctl() use the 32-bit ABI?  How would it
even know it's running on a 64-bit kernel?

> +[6] The implementation of the SHSTK
> +===================================
> +
> +SHSTK size
> +----------
> +
> +A task's SHSTK is allocated from memory to a fixed size of
> +RLIMIT_STACK.

I can't really parse that sentence.  Is this saying that shadow stacks
are limited by and share space with normal stacks via 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.

I thought the size was passed in from userspace?  Where does this sizing
take place?  Is this a convention or is it being enforced?

> +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.

						 ^ typo?

I'm not sure what this is trying to say.

> +The kernel creates a restore token at the SHSTK restoring address and
> +verifies that token when restoring from the signal handler.

I think there's a sentence or two of background missing here.  I'm
really lost as to what this is trying to tell me.

> +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.

I'm 100% lost.  I have no idea what this is trying to tell me or why it
is relevant to the kernel.

> +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.

What's the most important thing about shadow stacks and fork()?  Does
this documentation tell that to the reader?

> +When a pthread child is created, the kernel allocates a new SHSTK for
> +the new thread.

Why is this here?  Are pthread children created work fork()?

> +Setjmp/Longjmp
> +--------------
> +
> +Longjmp unwinds SHSTK until it matches the program stack.

I'm missing what this has to do with the kernel.

> +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

Nit: ARCH_X86_CET_ALLOC_SHSTK is not a syscall.

> +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.

This seems like a howto for doing user-level threading.  It seems like
it could be replaced by a single sentence in the
ARCH_X86_CET_ALLOC_SHSTK documentation explaining that new shadow stacks
are generally (always??) allocated along with new stacks.  Since new
clone() threads need a new stack, they also need a new shadow stack.
User-level threads that need a new stack are also expected to allocate a
new shadow stack.

Right?

> +[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.

I really don't think this belongs in the documentation, especially since
it's duplicated almost verbatim in code comments.

> +[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

								^ set up

> +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.