Re: [PATCH v9 10/12] x86, mpx: add prctl commands PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMEN

Thomas Gleixner
2014-10-24T12:49:53Z

On Sun, 12 Oct 2014, Qiaowei Ren wrote:
> +int mpx_enable_management(struct task_struct *tsk)
> +{
> + struct mm_struct *mm = tsk->mm;
> + void __user *bd_base = MPX_INVALID_BOUNDS_DIR;

What's the point of initializing bd_base here. I had to look twice to
figure out that it gets overwritten by task_get_bounds_dir()

> @@ -285,6 +285,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
> struct xsave_struct *xsave_buf;
> struct task_struct *tsk = current;
> siginfo_t info;
> + int ret = 0;
>
> prev_state = exception_enter();
> if (notify_die(DIE_TRAP, "bounds", regs, error_code,
> @@ -312,8 +313,35 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
> */
> switch (status & MPX_BNDSTA_ERROR_CODE) {
> case 2: /* Bound directory has invalid entry. */
> - if (do_mpx_bt_fault(xsave_buf))
> + down_write(&current->mm->mmap_sem);

The handling of mm->mmap_sem here is horrible. The only reason why you
want to hold mmap_sem write locked in the first place is that you want
to cover the allocation and the mm->bd_addr check.

I think it's wrong to tie this to mmap_sem in the first place. If MPX
is enabled then you should have mm->bd_addr and an explicit mutex to
protect it.

So the logic would look like this:

mutex_lock(&mm->bd_mutex);
if (!kernel_managed(mm))
do_trap();
else if (do_mpx_bt_fault())
force_sig();
mutex_unlock(&mm->bd_mutex);

No tricks with mmap_sem, no special return value handling. Straight
forward code instead of a convoluted and error prone mess.

Hmm?

Thanks,

tglx