Re: check *uaddr==val after queueing - without faulting

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-03-19 at 15:01 -0700, Darren Hart wrote:
> Adding a few key folks to the Cc, apologies for the short initial Cc list.
> 
> Darren Hart wrote:
> > The current futex_wait() code (I'm looking at tip/core/futexes)
> > conflicts with a warning in the comments about checking *uaddr==val
> > before the futex_q is queued on the hb list.  While userspace is able to
> > alter *uaddr at will and should expect to hang in the kernel forever
> > should it do so haphazardly, there are legitimate scenarios where the
> > futex value might change between the call to futex_wait() and when the
> > futex_q gets on the hb list.
> > 
> > For example, glibc protects access to the value of cond.__data.__futex
> > via the cond.__data.__lock.  However, before it can issue the syscall it
> > has to drop the cond.__data.__lock, leaving a small race window where
> > userspace might issue a signal or broadcast, which will modify the value
> > of cond.__data.__futex.  As I understand it, this will result in the
> > waiter having changed the value of the futex prior to entering the
> > kernel, but not enqueuing itself on the hb list until after the waiter
> > issues the broadcast that was intended to wake it up.
> > 
> > I was working up a patch to move the test to after the call to
> > queue_me(), but in order to do the test we also have to perform the
> > get_user() after the queue_me(), which might sleep if we still hold the
> > hb->lock.  If we let queue_me() drop the hb->lock before we call
> > get_user() then we may see a legitimate change in *uaddr that occured
> > after the queue_me() and before the get_user().
> > 
> > I'm at a loss for how to resolve the race without causing the false
> > positive inside the kernel.  It might be resolvable in glibc by looking
> > at the return code from futex_requeue and checking if the number 
> > woken_or_requeued agrees with the number it expected to be sleeping; 
> > this likely leaves other gaps for other waking calls, like FUTEX_WAKE.
> > 
> > Any thoughts?  Am I missing something that guards against this race?


get_user_pages_fast() the futex page, that will pin it, then under the
lock you can kmap_atomic() the page, and read it.

Probably massive overkill though :-)