On Mon, Jan 14, 2019 at 08:52:33AM -0800, Bart Van Assche wrote:
On Mon, 2019-01-14 at 13:52 +0100, Peter Zijlstra wrote:
On Fri, Jan 11, 2019 at 09:01:41AM -0800, Bart Van Assche wrote:
The list_del_rcu() call must only happen once.
Yes; obviously. But if we need to check all @pf's, that means the entry
is still reachable after a single reset_lock()/free_key_range(), which
is a bug.
I ran into complaints reporting that
the list_del_rcu() call triggered list corruption. This change made these complaints
disappear.
I'm saying this solution buggy, because that means the entry is still
reachable after we do call_rcu() (which is a straight up UAF).
Also put it differently, what guarantees checking those two @pf's is
sufficient. Suppose your earlier @pf already did the RCU callback and
freed stuff while the second is in progress. Then you're poking into
dead space.
zap_class() only examines elements of the list_entries[] array for which the
corresponding bit in list_entries_in_use has been set. The RCU callback clears
the bits in the list_entries_in_use that correspond to elements that have been
freed. The graph lock serializes zap_class() calls and the code inside the
RCU callback. So it's not clear to me why you are claiming that zap_class()
would trigger a use-after-free?
The scenario is like:
CPU0 CPU1 CPU2
lockdep_reset_lock_reg()
pf = get_pending_free_lock() // pf[0]
__lockdep_reset_lock(pf)
zap_class()
schedule_free_zapped_classes(pf)
call_rcu()
// here is wbere the objects 'freed' in zap_class()
// can still be used through references obtained
// __before__ we did call_rcu().
lockdep_reset_lock_reg()
pf = get_pending_free_lock() // pf[1]
__lockdep_reset_lock(pf)
zap_class()
list_entry_being_freed()
// checks: pf[0]
// this is a problem, it
// should _NEVER_ match
// anything from pf[0]
// those entries should
// be unreachable,
// otherwise:
rcu_read_lock()
entry = rcu_dereference()
<rcu-callback>
free_zapped_classes()
entry->class // UAF, just freed by rcu-callback
rcu_read_unlock()
Now, arguably, I'm having a really hard time actually finding the RCU user of
lock_list::entry, the comment in add_lock_to_list() seems to mention
look_up_lock_class(), but the only RCU usage there is the
lock_class::hash_entry, not lock_list::entry.
If lock_class is not indeed RCU used, that would simplify things. Please
double check.
But in any case, the normal RCU pattern is:
lock()
add-to-data-structure()
unlock()
rcu_read_lock()
obj = obtain-from-data-structure();
lock()
remove-from-data-structure()
call_rcu()
unlock();
use(obj);
rcu_read_unlock();
<rcu-callback>
actually-free-obj()
Fundamentally RCU delays the callback to the point where the last observer
that started before call_rcu() has finished and no later (in practise it often
is much later, but no guarantees there). So being able to reach an object
after you did call_rcu() on it is a fundamental fail.