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Merge branch 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 

Pull futex fix from Ingo Molnar:
 "A single fix for a robust futexes race between sys_exit() and
  sys_futex_lock_pi()"

* 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Cure exit race
This commit is contained in:
Linus Torvalds 2018-12-21 10:11:51 -08:00
parent 70ad6368e8 da791a6675
commit d5fa080d4c
1 changed files with 63 additions and 6 deletions
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69
kernel/futex.c
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@ -1148,11 +1148,65 @@ out_error:
return ret;
}
static int handle_exit_race(u32 __user *uaddr, u32 uval,
struct task_struct *tsk)
{
u32 uval2;
/*
* If PF_EXITPIDONE is not yet set, then try again.
*/
if (tsk && !(tsk->flags & PF_EXITPIDONE))
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->flags |= PF_EXITPIDONE; } else {
* if (!(tsk->flags & PF_EXITPIDONE))
* 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)
{
pid_t pid = uval & FUTEX_TID_MASK;
@ -1162,12 +1216,15 @@ 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 = find_get_task_by_vpid(pid);
if (!p)
return -ESRCH;
return handle_exit_race(uaddr, uval, NULL);
if (unlikely(p->flags & PF_KTHREAD)) {
put_task_struct(p);
@ -1187,7 +1244,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
* set, we know that the task has finished the
* cleanup:
*/
int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;
int ret = handle_exit_race(uaddr, uval, p);
raw_spin_unlock_irq(&p->pi_lock);
put_task_struct(p);
@ -1244,7 +1301,7 @@ 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);
return attach_to_pi_owner(uaddr, uval, key, ps);
}
static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
@ -1352,7 +1409,7 @@ 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);
return attach_to_pi_owner(uaddr, newval, key, ps);
}
/**