Re: [PATCH 01/10] x86/cet: User-mode shadow stack support

[Andy Lutomirski]

On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> This patch adds basic shadow stack enabling/disabling routines.
> A task's shadow stack is allocated from memory with VM_SHSTK
> flag set and read-only protection.  The shadow stack is
> allocated to a fixed size and that can be changed by the system
> admin.

How do threads work?  Can a user program mremap() its shadow stack to
make it bigger?

Also, did you add all the needed checks to make get_user_pages(),
access_process_vm(), etc fail when called on the shadow stack?  (Or at
least fail if they're requesting write access and the FORCE bit isn't
set.)

> +#define SHSTK_SIZE (0x8000 * (test_thread_flag(TIF_IA32) ? 4 : 8))

Please don't add more mode-dependent #defines.  Also, please try to
avoid adding any new code that looks at TIF_IA32 or similar.  Uses of
that bit are generally bugs, and the bit itself should get removed
some day.  If you need to make a guess, use in_compat_syscall() or
similar if appropriate.

> +
> +static inline int cet_set_shstk_ptr(unsigned long addr)
> +{
> +       u64 r;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +               return -1;
> +
> +       if ((addr >= TASK_SIZE) || (!IS_ALIGNED(addr, 4)))
> +               return -1;'

TASK_SIZE_MAX, please.  TASK_SIZE is weird and is usually the wrong
thing to use.

> +static unsigned long shstk_mmap(unsigned long addr, unsigned long len)
> +{
> +       struct mm_struct *mm = current->mm;
> +       unsigned long populate;
> +
> +       down_write(&mm->mmap_sem);
> +       addr = do_mmap(NULL, addr, len, PROT_READ,
> +                      MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK,
> +                      0, &populate, NULL);
> +       up_write(&mm->mmap_sem);
> +
> +       if (populate)
> +               mm_populate(addr, populate);

Please don't populate if do_mmap() failed.

> +int cet_setup_shstk(void)
> +{
> +       unsigned long addr, size;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +               return -EOPNOTSUPP;
> +
> +       size = SHSTK_SIZE;
> +       addr = shstk_mmap(0, size);
> +
> +       if (addr >= TASK_SIZE)
> +               return -ENOMEM;

Please check the actual value that do_mmap() would return on error.
(IS_ERR, 0, MAP_FAILED -- I don't remember.)

> +
> +       cet_set_shstk_ptr(addr + size - sizeof(void *));
> +       current->thread.cet.shstk_base = addr;
> +       current->thread.cet.shstk_size = size;
> +       current->thread.cet.shstk_enabled = 1;
> +       return 0;
> +}
> +
> +void cet_disable_shstk(void)
> +{
> +       u64 r;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +               return;
> +
> +       rdmsrl(MSR_IA32_U_CET, r);
> +       r &= ~(MSR_IA32_CET_SHSTK_EN);
> +       wrmsrl(MSR_IA32_U_CET, r);
> +       wrmsrl(MSR_IA32_PL3_SSP, 0);
> +       current->thread.cet.shstk_enabled = 0;
> +}
> +
> +void cet_disable_free_shstk(struct task_struct *tsk)
> +{
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
> +           !tsk->thread.cet.shstk_enabled)
> +               return;
> +
> +       if (tsk == current)
> +               cet_disable_shstk();

if tsk != current, then this will malfunction, right?  What is it
intended to do?

> +
> +       /*
> +        * Free only when tsk is current or shares mm
> +        * with current but has its own shstk.
> +        */
> +       if (tsk->mm && (tsk->mm == current->mm) &&
> +           (tsk->thread.cet.shstk_base)) {
> +               vm_munmap(tsk->thread.cet.shstk_base,
> +                         tsk->thread.cet.shstk_size);
> +               tsk->thread.cet.shstk_base = 0;
> +               tsk->thread.cet.shstk_size = 0;
> +       }

I'm having trouble imagining why the kernel would ever want to
automatically free the shadow stack vma.  What is this for?