diff options
| author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2016-04-15 14:35:39 +0200 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2016-05-04 14:48:43 -0700 |
| commit | 61fc0ae42c498f8eb782733065d93da6817d28b4 (patch) | |
| tree | 223a0555841932d315a855b06a64fd9998ae0a2d /kernel/futex.c | |
| parent | 4fad26a279caad671287e502e9b6d2487a56b270 (diff) | |
futex: Handle unlock_pi race gracefully
commit 89e9e66ba1b3bde9d8ea90566c2aee20697ad681 upstream.
If userspace calls UNLOCK_PI unconditionally without trying the TID -> 0
transition in user space first then the user space value might not have the
waiters bit set. This opens the following race:
CPU0 CPU1
uval = get_user(futex)
lock(hb)
lock(hb)
futex |= FUTEX_WAITERS
....
unlock(hb)
cmpxchg(futex, uval, newval)
So the cmpxchg fails and returns -EINVAL to user space, which is wrong because
the futex value is valid.
To handle this (yes, yet another) corner case gracefully, check for a flag
change and retry.
[ tglx: Massaged changelog and slightly reworked implementation ]
Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1460723739-5195-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel/futex.c')
| -rw-r--r-- | kernel/futex.c | 25 |
1 files changed, 22 insertions, 3 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 461c72b2dac2..eaa3a8dfd345 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1244,10 +1244,20 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this, if (unlikely(should_fail_futex(true))) ret = -EFAULT; - if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) + if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { ret = -EFAULT; - else if (curval != uval) - ret = -EINVAL; + } else if (curval != uval) { + /* + * If a unconditional UNLOCK_PI operation (user space did not + * try the TID->0 transition) raced with a waiter setting the + * FUTEX_WAITERS flag between get_user() and locking the hash + * bucket lock, retry the operation. + */ + if ((FUTEX_TID_MASK & curval) == uval) + ret = -EAGAIN; + else + ret = -EINVAL; + } if (ret) { raw_spin_unlock(&pi_state->pi_mutex.wait_lock); return ret; @@ -2538,6 +2548,15 @@ retry: if (ret == -EFAULT) goto pi_faulted; /* + * A unconditional UNLOCK_PI op raced against a waiter + * setting the FUTEX_WAITERS bit. Try again. + */ + if (ret == -EAGAIN) { + spin_unlock(&hb->lock); + put_futex_key(&key); + goto retry; + } + /* * wake_futex_pi has detected invalid state. Tell user * space. */ |