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
