From: Peter Zijlstra peterz@infradead.org

Currently futex-pi relies on hb->lock to serialize everything. But hb->lock creates another set of problems, especially priority inversions on RT where hb->lock becomes a rt_mutex itself.

The rt_mutex::wait_lock is the most obvious protection for keeping the futex user space value and the kernel internal pi_state in sync.

Rework and document the locking so rt_mutex::wait_lock is held accross all operations which modify the user space value and the pi state.

This allows to invoke rt_mutex_unlock() (including deboost) without holding hb->lock as a next step.

Nothing yet relies on the new locking rules.

Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104151.751993333@infradead.org Signed-off-by: Thomas Gleixner tglx@linutronix.de [Lee: Back-ported in support of a previous futex back-port attempt] Signed-off-by: Lee Jones lee.jones@linaro.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Zheng Yejian zhengyejian1@huawei.com --- kernel/futex.c | 138 +++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 112 insertions(+), 26 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c index a14b7ef90e5c..b410752f5ad1 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1014,6 +1014,39 @@ static void exit_pi_state_list(struct task_struct *curr) * [10] There is no transient state which leaves owner and user space * TID out of sync. Except one error case where the kernel is denied * write access to the user address, see fixup_pi_state_owner(). + * + * + * Serialization and lifetime rules: + * + * hb->lock: + * + * hb -> futex_q, relation + * futex_q -> pi_state, relation + * + * (cannot be raw because hb can contain arbitrary amount + * of futex_q's) + * + * pi_mutex->wait_lock: + * + * {uval, pi_state} + * + * (and pi_mutex 'obviously') + * + * p->pi_lock: + * + * p->pi_state_list -> pi_state->list, relation + * + * pi_state->refcount: + * + * pi_state lifetime + * + * + * Lock order: + * + * hb->lock + * pi_mutex->wait_lock + * p->pi_lock + * */

/* @@ -1021,10 +1054,12 @@ static void exit_pi_state_list(struct task_struct *curr) * the pi_state against the user space value. If correct, attach to * it. */ -static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, +static int attach_to_pi_state(u32 __user *uaddr, u32 uval, + struct futex_pi_state *pi_state, struct futex_pi_state **ps) { pid_t pid = uval & FUTEX_TID_MASK; + int ret, uval2;

/* * Userspace might have messed up non-PI and PI futexes [3] @@ -1032,8 +1067,33 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, if (unlikely(!pi_state)) return -EINVAL;

+ /* + * We get here with hb->lock held, and having found a + * futex_top_waiter(). This means that futex_lock_pi() of said futex_q + * has dropped the hb->lock in between queue_me() and unqueue_me_pi(), + * which in turn means that futex_lock_pi() still has a reference on + * our pi_state. + */ WARN_ON(!atomic_read(&pi_state->refcount));

+ /* + * Now that we have a pi_state, we can acquire wait_lock + * and do the state validation. + */ + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); + + /* + * Since {uval, pi_state} is serialized by wait_lock, and our current + * uval was read without holding it, it can have changed. Verify it + * still is what we expect it to be, otherwise retry the entire + * operation. + */ + if (get_futex_value_locked(&uval2, uaddr)) + goto out_efault; + + if (uval != uval2) + goto out_eagain; + /* * Handle the owner died case: */ @@ -1049,11 +1109,11 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * is not 0. Inconsistent state. [5] */ if (pid) - return -EINVAL; + goto out_einval; /* * Take a ref on the state and return success. [4] */ - goto out_state; + goto out_attach; }

/* @@ -1065,14 +1125,14 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * Take a ref on the state and return success. [6] */ if (!pid) - goto out_state; + goto out_attach; } else { /* * If the owner died bit is not set, then the pi_state * must have an owner. [7] */ if (!pi_state->owner) - return -EINVAL; + goto out_einval; }

/* @@ -1081,11 +1141,29 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * user space TID. [9/10] */ if (pid != task_pid_vnr(pi_state->owner)) - return -EINVAL; -out_state: + goto out_einval; + +out_attach: atomic_inc(&pi_state->refcount); + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); *ps = pi_state; return 0; + +out_einval: + ret = -EINVAL; + goto out_error; + +out_eagain: + ret = -EAGAIN; + goto out_error; + +out_efault: + ret = -EFAULT; + goto out_error; + +out_error: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); + return ret; }

/** @@ -1178,6 +1256,9 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,

/* * No existing pi state. First waiter. [2] + * + * This creates pi_state, we have hb->lock held, this means nothing can + * observe this state, wait_lock is irrelevant. */ pi_state = alloc_pi_state();

@@ -1202,7 +1283,8 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, return 0; }

-static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, +static int lookup_pi_state(u32 __user *uaddr, u32 uval, + struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps, struct task_struct **exiting) { @@ -1213,7 +1295,7 @@ static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, * attach to the pi_state when the validation succeeds. */ if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps);

/* * We are the first waiter - try to look up the owner based on @@ -1232,7 +1314,7 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))) return -EFAULT;

- /*If user space value changed, let the caller retry */ + /* If user space value changed, let the caller retry */ return curval != uval ? -EAGAIN : 0; }

@@ -1296,7 +1378,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, */ match = futex_top_waiter(hb, key); if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps);

/* * No waiter and user TID is 0. We are here because the @@ -1436,6 +1518,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,

if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { ret = -EFAULT; + } else if (curval != uval) { /* * If a unconditional UNLOCK_PI operation (user space did not @@ -1969,7 +2052,7 @@ retry_private: * rereading and handing potential crap to * lookup_pi_state. */ - ret = lookup_pi_state(ret, hb2, &key2, + ret = lookup_pi_state(uaddr2, ret, hb2, &key2, &pi_state, &exiting); }

@@ -2247,7 +2330,6 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, int err = 0;

oldowner = pi_state->owner; - /* * We are here because either: * @@ -2266,11 +2348,10 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, * because we can fault here. Imagine swapped out pages or a fork * that marked all the anonymous memory readonly for cow. * - * Modifying pi_state _before_ the user space value would - * leave the pi_state in an inconsistent state when we fault - * here, because we need to drop the hash bucket lock to - * handle the fault. This might be observed in the PID check - * in lookup_pi_state. + * Modifying pi_state _before_ the user space value would leave the + * pi_state in an inconsistent state when we fault here, because we + * need to drop the locks to handle the fault. This might be observed + * in the PID check in lookup_pi_state. */ retry: if (!argowner) { @@ -2331,21 +2412,26 @@ retry: return argowner == current;

/* - * To handle the page fault we need to drop the hash bucket - * lock here. That gives the other task (either the highest priority - * waiter itself or the task which stole the rtmutex) the - * chance to try the fixup of the pi_state. So once we are - * back from handling the fault we need to check the pi_state - * after reacquiring the hash bucket lock and before trying to - * do another fixup. When the fixup has been done already we - * simply return. + * To handle the page fault we need to drop the locks here. That gives + * the other task (either the highest priority waiter itself or the + * task which stole the rtmutex) the chance to try the fixup of the + * pi_state. So once we are back from handling the fault we need to + * check the pi_state after reacquiring the locks and before trying to + * do another fixup. When the fixup has been done already we simply + * return. + * + * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely + * drop hb->lock since the caller owns the hb -> futex_q relation. + * Dropping the pi_mutex->wait_lock requires the state revalidate. */ handle_fault: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(q->lock_ptr);

err = fault_in_user_writeable(uaddr);

spin_lock(q->lock_ptr); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

/* * Check if someone else fixed it for us:

From: Thomas Gleixner tglx@linutronix.de

commit da791a667536bf8322042e38ca85d55a78d3c273 upstream.

Stefan reported, that the glibc tst-robustpi4 test case fails occasionally. That case creates the following race between sys_exit() and sys_futex_lock_pi():

CPU0 CPU1

sys_exit() sys_futex() do_exit() futex_lock_pi() exit_signals(tsk) No waiters: tsk->flags |= PF_EXITING; *uaddr == 0x00000PID mm_release(tsk) Set waiter bit exit_robust_list(tsk) { *uaddr = 0x80000PID; Set owner died attach_to_pi_owner() { *uaddr = 0xC0000000; tsk = get_task(PID); } if (!tsk->flags & PF_EXITING) { ... attach(); tsk->flags |= PF_EXITPIDONE; } else { if (!(tsk->flags & PF_EXITPIDONE)) return -EAGAIN; return -ESRCH; <--- FAIL }

ESRCH is returned all the way to user space, which triggers the glibc test case assert. Returning ESRCH unconditionally is wrong here because the user space value has been changed by the exiting task to 0xC0000000, i.e. the FUTEX_OWNER_DIED bit is set and the futex PID value has been cleared. This is a valid state and the kernel has to handle it, i.e. taking the futex.

Cure it by rereading the user space value when PF_EXITING and PF_EXITPIDONE is set in the task which 'owns' the futex. If the value has changed, let the kernel retry the operation, which includes all regular sanity checks and correctly handles the FUTEX_OWNER_DIED case.

If it hasn't changed, then return ESRCH as there is no way to distinguish this case from malfunctioning user space. This happens when the exiting task did not have a robust list, the robust list was corrupted or the user space value in the futex was simply bogus.

Reported-by: Stefan Liebler stli@linux.ibm.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Acked-by: Peter Zijlstra peterz@infradead.org Cc: Heiko Carstens heiko.carstens@de.ibm.com Cc: Darren Hart dvhart@infradead.org Cc: Ingo Molnar mingo@kernel.org Cc: Sasha Levin sashal@kernel.org Cc: stable@vger.kernel.org Link: https://bugzilla.kernel.org/show_bug.cgi?id=200467 Link: https://lkml.kernel.org/r/20181210152311.986181245@linutronix.de Signed-off-by: Sudip Mukherjee sudipm.mukherjee@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [Lee: Required to satisfy functional dependency from futex back-port. Re-add the missing handle_exit_race() parts from: 3d4775df0a89 ("futex: Replace PF_EXITPIDONE with a state")] Signed-off-by: Lee Jones lee.jones@linaro.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Zheng Yejian zhengyejian1@huawei.com --- kernel/futex.c | 71 +++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 65 insertions(+), 6 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c index b410752f5ad1..116766ef7de6 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1196,11 +1196,67 @@ static void wait_for_owner_exiting(int ret, struct task_struct *exiting) put_task_struct(exiting); }

+static int handle_exit_race(u32 __user *uaddr, u32 uval, + struct task_struct *tsk) +{ + u32 uval2; + + /* + * If the futex exit state is not yet FUTEX_STATE_DEAD, wait + * for it to finish. + */ + if (tsk && tsk->futex_state != FUTEX_STATE_DEAD) + return -EAGAIN; + + /* + * Reread the user space value to handle the following situation: + * + * CPU0 CPU1 + * + * sys_exit() sys_futex() + * do_exit() futex_lock_pi() + * futex_lock_pi_atomic() + * exit_signals(tsk) No waiters: + * tsk->flags |= PF_EXITING; *uaddr == 0x00000PID + * mm_release(tsk) Set waiter bit + * exit_robust_list(tsk) { *uaddr = 0x80000PID; + * Set owner died attach_to_pi_owner() { + * *uaddr = 0xC0000000; tsk = get_task(PID); + * } if (!tsk->flags & PF_EXITING) { + * ... attach(); + * tsk->futex_state = } else { + * FUTEX_STATE_DEAD; if (tsk->futex_state != + * FUTEX_STATE_DEAD) + * return -EAGAIN; + * return -ESRCH; <--- FAIL + * } + * + * Returning ESRCH unconditionally is wrong here because the + * user space value has been changed by the exiting task. + * + * The same logic applies to the case where the exiting task is + * already gone. + */ + if (get_futex_value_locked(&uval2, uaddr)) + return -EFAULT; + + /* If the user space value has changed, try again. */ + if (uval2 != uval) + return -EAGAIN; + + /* + * The exiting task did not have a robust list, the robust list was + * corrupted or the user space value in *uaddr is simply bogus. + * Give up and tell user space. + */ + return -ESRCH; +} + /* * Lookup the task for the TID provided from user space and attach to * it after doing proper sanity checks. */ -static int attach_to_pi_owner(u32 uval, union futex_key *key, +static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key, struct futex_pi_state **ps, struct task_struct **exiting) { @@ -1211,12 +1267,15 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, /* * We are the first waiter - try to look up the real owner and attach * the new pi_state to it, but bail out when TID = 0 [1] + * + * The !pid check is paranoid. None of the call sites should end up + * with pid == 0, but better safe than sorry. Let the caller retry */ if (!pid) - return -ESRCH; + return -EAGAIN; p = futex_find_get_task(pid); if (!p) - return -ESRCH; + return handle_exit_race(uaddr, uval, NULL);

if (unlikely(p->flags & PF_KTHREAD)) { put_task_struct(p); @@ -1235,7 +1294,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, * FUTEX_STATE_DEAD, we know that the task has finished * the cleanup: */ - int ret = (p->futex_state = FUTEX_STATE_DEAD) ? -ESRCH : -EAGAIN; + int ret = handle_exit_race(uaddr, uval, p);

raw_spin_unlock_irq(&p->pi_lock); /* @@ -1301,7 +1360,7 @@ static int lookup_pi_state(u32 __user *uaddr, u32 uval, * We are the first waiter - try to look up the owner based on * @uval and attach to it. */ - return attach_to_pi_owner(uval, key, ps, exiting); + return attach_to_pi_owner(uaddr, uval, key, ps, exiting); }

static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) @@ -1417,7 +1476,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, * attach to the owner. If that fails, no harm done, we only * set the FUTEX_WAITERS bit in the user space variable. */ - return attach_to_pi_owner(uval, key, ps, exiting); + return attach_to_pi_owner(uaddr, newval, key, ps, exiting); }

/**