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

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