Re: [PATCH v2 01/39] Documentation/x86: Add CET description

[Kees Cook]

On Thu, Sep 29, 2022 at 03:28:58PM -0700, Rick Edgecombe wrote:
> [...]
> +Overview
> +========
> +
> +Control-flow Enforcement Technology (CET) is term referring to several
> +related x86 processor features that provides protection against control
> +flow hijacking attacks. The HW feature itself can be set up to protect
> +both applications and the kernel. Only user-mode protection is implemented
> +in the 64-bit kernel.

This likely needs rewording, since it's not strictly true any more:
IBT is supported in kernel-mode now (CONFIG_X86_IBT).

> +CET introduces Shadow Stack and Indirect Branch Tracking. Shadow stack is
> +a secondary stack allocated from memory and 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. Indirect branch tracking 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
> +and only Shadow Stack is currently supported in the kernel.
> +
> +The Kconfig options is X86_SHADOW_STACK, and it can be disabled with
> +the kernel parameter clearcpuid, like this: "clearcpuid=shstk".
> +
> +To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or LLVM v10.0.1
> +or later are required. To build a CET-enabled application, GLIBC v2.28 or
> +later is also required.
> +
> +At run time, /proc/cpuinfo shows CET features if the processor supports
> +CET.

Maybe call them out by name: shstk ibt

> +CET arch_prctl()'s
> +==================
> +
> +Elf features should be enabled by the loader using the below arch_prctl's.
> +
> +arch_prctl(ARCH_CET_ENABLE, unsigned int feature)
> +    Enable a single feature specified in 'feature'. Can only operate on
> +    one feature at a time.

Does this mean only 1 bit out of the 32 may be specified?

> +
> +arch_prctl(ARCH_CET_DISABLE, unsigned int feature)
> +    Disable features specified in 'feature'. Can only operate on
> +    one feature at a time.
> +
> +arch_prctl(ARCH_CET_LOCK, unsigned int features)
> +    Lock in features at their current enabled or disabled status.

How is the "features" argument processed here?

> [...]
> +Proc status
> +===========
> +To check if an application is actually running with shadow stack, the
> +user can read the /proc/$PID/arch_status. It will report "wrss" or
> +"shstk" depending on what is enabled.

TIL about "arch_status". :) Why is this a separate file? "status" is
already has unique field names.

> +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.

Perhaps speak to the ASLR characteristics of the shstk here?

Also, it seems if there is a "Fork" section, there should be an "Exec"
section? I suspect it would be short: shstk is disabled when execve() is
called and must be re-enabled from userspace, yes?

-Kees

-- 
Kees Cook