aboutsummaryrefslogtreecommitdiff
path: root/kernel
diff options
context:
space:
mode:
authorSebastian Andrzej Siewior <bigeasy@linutronix.de>2016-04-15 14:35:39 +0200
committerSasha Levin <sasha.levin@oracle.com>2016-05-10 12:20:17 -0400
commit561c943e1709fe80588f21bdc2e2badc69540ead (patch)
treedf9feb8e9a46f796e40ddacd10c759a00183e5ae /kernel
parent0a3c8132816028ad6870dd89a149e68f7f3f7f86 (diff)
downloadlinux-linaro-stable-561c943e1709fe80588f21bdc2e2badc69540ead.tar.gz
futex: Handle unlock_pi race gracefully
[ Upstream commit 89e9e66ba1b3bde9d8ea90566c2aee20697ad681 ] 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: stable@vger.kernel.org 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: Sasha Levin <sasha.levin@oracle.com>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/futex.c25
1 files changed, 22 insertions, 3 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index d9d63806f55f..d58859d62b8b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1157,10 +1157,20 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
*/
newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
- 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;
@@ -2419,6 +2429,15 @@ 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;
+ }
goto out_unlock;
}