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.
