1. 17 Jul, 2014 39 commits
  2. 16 Jul, 2014 1 commit
    • Thomas Gleixner's avatar
      rtmutex: Plug slow unlock race · 0c57bb13
      Thomas Gleixner authored
      commit 27e35715 upstream.
      
      When the rtmutex fast path is enabled the slow unlock function can
      create the following situation:
      
      spin_lock(foo->m->wait_lock);
      foo->m->owner = NULL;
      	    			rt_mutex_lock(foo->m); <-- fast path
      				free = atomic_dec_and_test(foo->refcnt);
      				rt_mutex_unlock(foo->m); <-- fast path
      				if (free)
      				   kfree(foo);
      
      spin_unlock(foo->m->wait_lock); <--- Use after free.
      
      Plug the race by changing the slow unlock to the following scheme:
      
           while (!rt_mutex_has_waiters(m)) {
           	    /* Clear the waiters bit in m->owner */
      	    clear_rt_mutex_waiters(m);
            	    owner = rt_mutex_owner(m);
            	    spin_unlock(m->wait_lock);
            	    if (cmpxchg(m->owner, owner, 0) == owner)
            	       return;
            	    spin_lock(m->wait_lock);
           }
      
      So in case of a new waiter incoming while the owner tries the slow
      path unlock we have two situations:
      
       unlock(wait_lock);
      					lock(wait_lock);
       cmpxchg(p, owner, 0) == owner
       	    	   			mark_rt_mutex_waiters(lock);
      	 				acquire(lock);
      
      Or:
      
       unlock(wait_lock);
      					lock(wait_lock);
      	 				mark_rt_mutex_waiters(lock);
       cmpxchg(p, owner, 0) != owner
      					enqueue_waiter();
      					unlock(wait_lock);
       lock(wait_lock);
       wakeup_next waiter();
       unlock(wait_lock);
      					lock(wait_lock);
      					acquire(lock);
      
      If the fast path is disabled, then the simple
      
         m->owner = NULL;
         unlock(m->wait_lock);
      
      is sufficient as all access to m->owner is serialized via
      m->wait_lock;
      
      Also document and clarify the wakeup_next_waiter function as suggested
      by Oleg Nesterov.
      Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Reviewed-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      Cc: Peter Zijlstra <peterz@infradead.org>
      Link: http://lkml.kernel.org/r/20140611183852.937945560@linutronix.deSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
      Signed-off-by: default avatarMike Galbraith <umgwanakikbuti@gmail.com>
      Signed-off-by: default avatarJiri Slaby <jslaby@suse.cz>
      0c57bb13