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authorPeter Zijlstra <peterz@infradead.org>2017-12-08 13:49:39 +0100
committerThomas Gleixner <tglx@linutronix.de>2018-01-14 18:49:16 +0100
commitc1e2f0eaf015fb7076d51a339011f2383e6dd389 (patch)
treec993bc9680bf3297d147c1bae503e6e47362bad7 /kernel/futex.c
parentc92a9a461dff6140c539c61e457aa97df29517d6 (diff)
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futex: Avoid violating the 10th rule of futex
Julia reported futex state corruption in the following scenario: waiter waker stealer (prio > waiter) futex(WAIT_REQUEUE_PI, uaddr, uaddr2, timeout=[N ms]) futex_wait_requeue_pi() futex_wait_queue_me() freezable_schedule() <scheduled out> futex(LOCK_PI, uaddr2) futex(CMP_REQUEUE_PI, uaddr, uaddr2, 1, 0) /* requeues waiter to uaddr2 */ futex(UNLOCK_PI, uaddr2) wake_futex_pi() cmp_futex_value_locked(uaddr2, waiter) wake_up_q() <woken by waker> <hrtimer_wakeup() fires, clears sleeper->task> futex(LOCK_PI, uaddr2) __rt_mutex_start_proxy_lock() try_to_take_rt_mutex() /* steals lock */ rt_mutex_set_owner(lock, stealer) <preempted> <scheduled in> rt_mutex_wait_proxy_lock() __rt_mutex_slowlock() try_to_take_rt_mutex() /* fails, lock held by stealer */ if (timeout && !timeout->task) return -ETIMEDOUT; fixup_owner() /* lock wasn't acquired, so, fixup_pi_state_owner skipped */ return -ETIMEDOUT; /* At this point, we've returned -ETIMEDOUT to userspace, but the * futex word shows waiter to be the owner, and the pi_mutex has * stealer as the owner */ futex_lock(LOCK_PI, uaddr2) -> bails with EDEADLK, futex word says we're owner. And suggested that what commit: 73d786bd043e ("futex: Rework inconsistent rt_mutex/futex_q state") removes from fixup_owner() looks to be just what is needed. And indeed it is -- I completely missed that requeue_pi could also result in this case. So we need to restore that, except that subsequent patches, like commit: 16ffa12d7425 ("futex: Pull rt_mutex_futex_unlock() out from under hb->lock") changed all the locking rules. Even without that, the sequence: - if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) { - locked = 1; - goto out; - } - raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock); - owner = rt_mutex_owner(&q->pi_state->pi_mutex); - if (!owner) - owner = rt_mutex_next_owner(&q->pi_state->pi_mutex); - raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock); - ret = fixup_pi_state_owner(uaddr, q, owner); already suggests there were races; otherwise we'd never have to look at next_owner. So instead of doing 3 consecutive wait_lock sections with who knows what races, we do it all in a single section. Additionally, the usage of pi_state->owner in fixup_owner() was only safe because only the rt_mutex owner would modify it, which this additional case wrecks. Luckily the values can only change away and not to the value we're testing, this means we can do a speculative test and double check once we have the wait_lock. Fixes: 73d786bd043e ("futex: Rework inconsistent rt_mutex/futex_q state") Reported-by: Julia Cartwright <julia@ni.com> Reported-by: Gratian Crisan <gratian.crisan@ni.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Julia Cartwright <julia@ni.com> Tested-by: Gratian Crisan <gratian.crisan@ni.com> Cc: Darren Hart <dvhart@infradead.org> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20171208124939.7livp7no2ov65rrc@hirez.programming.kicks-ass.net
Diffstat (limited to 'kernel/futex.c')
-rw-r--r--kernel/futex.c83
1 files changed, 67 insertions, 16 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 57d0b3657e16..9e69589b9248 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2294,21 +2294,17 @@ static void unqueue_me_pi(struct futex_q *q)
spin_unlock(q->lock_ptr);
}
-/*
- * Fixup the pi_state owner with the new owner.
- *
- * Must be called with hash bucket lock held and mm->sem held for non
- * private futexes.
- */
static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
- struct task_struct *newowner)
+ struct task_struct *argowner)
{
- u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
struct futex_pi_state *pi_state = q->pi_state;
u32 uval, uninitialized_var(curval), newval;
- struct task_struct *oldowner;
+ struct task_struct *oldowner, *newowner;
+ u32 newtid;
int ret;
+ lockdep_assert_held(q->lock_ptr);
+
raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
oldowner = pi_state->owner;
@@ -2317,11 +2313,17 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
newtid |= FUTEX_OWNER_DIED;
/*
- * We are here either because we stole the rtmutex from the
- * previous highest priority waiter or we are the highest priority
- * waiter but have failed to get the rtmutex the first time.
+ * We are here because either:
+ *
+ * - we stole the lock and pi_state->owner needs updating to reflect
+ * that (@argowner == current),
*
- * We have to replace the newowner TID in the user space variable.
+ * or:
+ *
+ * - someone stole our lock and we need to fix things to point to the
+ * new owner (@argowner == NULL).
+ *
+ * Either way, we have to replace the TID in the user space variable.
* This must be atomic as we have to preserve the owner died bit here.
*
* Note: We write the user space value _before_ changing the pi_state
@@ -2334,6 +2336,42 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
* in the PID check in lookup_pi_state.
*/
retry:
+ if (!argowner) {
+ if (oldowner != current) {
+ /*
+ * We raced against a concurrent self; things are
+ * already fixed up. Nothing to do.
+ */
+ ret = 0;
+ goto out_unlock;
+ }
+
+ if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
+ /* We got the lock after all, nothing to fix. */
+ ret = 0;
+ goto out_unlock;
+ }
+
+ /*
+ * Since we just failed the trylock; there must be an owner.
+ */
+ newowner = rt_mutex_owner(&pi_state->pi_mutex);
+ BUG_ON(!newowner);
+ } else {
+ WARN_ON_ONCE(argowner != current);
+ if (oldowner == current) {
+ /*
+ * We raced against a concurrent self; things are
+ * already fixed up. Nothing to do.
+ */
+ ret = 0;
+ goto out_unlock;
+ }
+ newowner = argowner;
+ }
+
+ newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+
if (get_futex_value_locked(&uval, uaddr))
goto handle_fault;
@@ -2434,9 +2472,9 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
* Got the lock. We might not be the anticipated owner if we
* did a lock-steal - fix up the PI-state in that case:
*
- * We can safely read pi_state->owner without holding wait_lock
- * because we now own the rt_mutex, only the owner will attempt
- * to change it.
+ * Speculative pi_state->owner read (we don't hold wait_lock);
+ * since we own the lock pi_state->owner == current is the
+ * stable state, anything else needs more attention.
*/
if (q->pi_state->owner != current)
ret = fixup_pi_state_owner(uaddr, q, current);
@@ -2444,6 +2482,19 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
}
/*
+ * If we didn't get the lock; check if anybody stole it from us. In
+ * that case, we need to fix up the uval to point to them instead of
+ * us, otherwise bad things happen. [10]
+ *
+ * Another speculative read; pi_state->owner == current is unstable
+ * but needs our attention.
+ */
+ if (q->pi_state->owner == current) {
+ ret = fixup_pi_state_owner(uaddr, q, NULL);
+ goto out;
+ }
+
+ /*
* Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex.
*/