• Peter Zijlstra's avatar
    futex: Fix more put_pi_state() vs. exit_pi_state_list() races · 153fbd12
    Peter Zijlstra authored
    Dmitry (through syzbot) reported being able to trigger the WARN in
    get_pi_state() and a use-after-free on:
    
    	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
    
    Both are due to this race:
    
      exit_pi_state_list()				put_pi_state()
    
      lock(&curr->pi_lock)
      while() {
    	pi_state = list_first_entry(head);
    	hb = hash_futex(&pi_state->key);
    	unlock(&curr->pi_lock);
    
    						dec_and_test(&pi_state->refcount);
    
    	lock(&hb->lock)
    	lock(&pi_state->pi_mutex.wait_lock)	// uaf if pi_state free'd
    	lock(&curr->pi_lock);
    
    	....
    
    	unlock(&curr->pi_lock);
    	get_pi_state();				// WARN; refcount==0
    
    The problem is we take the reference count too late, and don't allow it
    being 0. Fix it by using inc_not_zero() and simply retrying the loop
    when we fail to get a refcount. In that case put_pi_state() should
    remove the entry from the list.
    Reported-by: default avatarDmitry Vyukov <dvyukov@google.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Gratian Crisan <gratian.crisan@ni.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: dvhart@infradead.org
    Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>
    Cc: syzkaller-bugs@googlegroups.com
    Cc: <stable@vger.kernel.org>
    Fixes: c74aef2d ("futex: Fix pi_state->owner serialization")
    Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@hirez.programming.kicks-ass.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    153fbd12
futex.c 96.3 KB