On 03/28/2015 01:39 AM, Borislav Petkov wrote:
> AFAICT, in this case, we return only a 32-bit value and don't touch
> the upper 32 bits of actual_old_val which might be a problem if the
> assumptions of the callers is that the whole unsigned long is being
> changed.
The suggestion to just drop in the futex code does not work for just
that reason.
We do this:
static int unmap_single_bt(struct mm_struct *mm,
{
...
unsigned long uninitialized_var(actual_old_val);
ret = mpx_cmpxchg_bd_entry(mm, &actual_old_val,
bd_entry, bt_addr, cleared_bd_entry);
and then check:
if (actual_old_val != expected_old_val) {
If we do not touch the upper 32-bits of 'actual_old_val', then we might
end up with stack gunk in there. The other caller of
mpx_cmpxchg_bd_entry() is OK since it initializes its 'actual_old_val'.
So, I don't think it will work as you've written. We need to somehow
ensure that the upper 32-bits match the upper 32-bits of
'expected_old_val' which will always be 0's for a 32-bit app.
So, yeah, it's ugly. You got me. But all the 64/32-bit conversions are
done out in the open and it's obvious what's going on. It is also
_tested_ and works.
I'd really like to keep it the way it is.
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