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

[Yu-cheng Yu]

On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote:
> 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?

A pthread's shadow stack is allocated/freed by the kernel.  This patch
has the supporting routines that handle both non-pthread and pthread.

In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread
shadow stack in copy_thread_tls(), and free it in deactivate_mm().

If clone of a pthread fails, shadow stack is freed in
cet_disable_free_shstk() below (I will add more comments):

If (Current thread existing)
	Disable and free shadow stack

If (Clone of a pthread fails)
	Free the pthread shadow stack

We block mremap, mprotect, madvise, and munmap on a vma that has
VM_SHSTK (in separate patches).

> 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.)

Currently if FORCE bit is set, these functions can write to shadow
stack, otherwise write access will fail.  I will test it.

> > +#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.

OK.

> 
> > +
> > +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.

OK.

> 
> > +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.

I will fix it.

> 
> > +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.)

OK.

> 
> > +
> > +       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?

We get here when clone fails.  In this condition, we don't disable the
calling task's shadow stack.  I will add comments.

> 
> > +
> > +       /*
> > +        * 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?

This is for pthreads.  When a pthread exits, its shadow stack needs to
be freed.