linux-kernel - Re: [PATCH] ipc/sem.c: synchronize semop and semctl with IPC

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Message-ID: <1380733448.2313.6.camel@buesod1.americas.hpqcorp.net>
Date:	Wed, 02 Oct 2013 10:04:08 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] ipc/sem.c: synchronize semop and semctl with IPC_RMID

On Tue, 2013-10-01 at 06:22 +0200, Manfred Spraul wrote:
> Hi Davidlohr,
> 
> On 09/30/2013 07:54 PM, Davidlohr Bueso wrote:
> > Hi Manfred,
> >
> > On Mon, 2013-09-30 at 11:13 +0200, Manfred Spraul wrote:
> >> After acquiring the semlock spinlock, the operations must test that the
> >> array is still valid.
> >>
> >> - semctl() and exit_sem() would walk stale linked lists (ugly, but should
> >>    be ok: all lists are empty)
> >>
> >> - semtimedop() would sleep forever - and if woken up due to a signal -
> >>    access memory after free.
> > Yep, that was next on my list - I had a patch for semtimedop() but was
> > waiting to rebase it on top of your previous changes. Anyway thanks for
> > sending this.
> >
> >> The patch standardizes the tests for .deleted, so that all tests in one
> >> function leave the function with the same approach.
> >>
> >> Right now, it's a mixture of "goto cleanup", some cleanup and then
> >> "goto further_cleanup" and all cleanup+"return -EIDRM" - that makes the
> >> review much harder.
> >>
> >> Davidlohr: Could you please review the patch?
> >> I did some stress test, but probably I didn't hit exactly the modified
> >> lines.
> > This shouldn't affect performance, if that's what you mean.
> All goto's must go to the correct target, free everything, unlock 
> everything, do not unlock twice, ...
> 
> >   One more
> > read in the critical region won't make any difference. The patch looks
> > good, just one doubt below.
> >
> >
> >> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
> >> ---
> >>   ipc/sem.c | 43 ++++++++++++++++++++++++++++++-------------
> >>   1 file changed, 30 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/ipc/sem.c b/ipc/sem.c
> >> index 19c8b98..a2fa795 100644
> >> --- a/ipc/sem.c
> >> +++ b/ipc/sem.c
> >> @@ -1229,6 +1229,12 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
> >>   
> >>   	sem_lock(sma, NULL, -1);
> >>   
> >> +	if (sma->sem_perm.deleted) {
> >> +		sem_unlock(sma, -1);
> >> +		rcu_read_unlock();
> >> +		return -EIDRM;
> >> +	}
> >> +
> >>   	curr = &sma->sem_base[semnum];
> >>   
> >>   	ipc_assert_locked_object(&sma->sem_perm);
> >> @@ -1285,10 +1291,8 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> >>   		sem_lock(sma, NULL, -1);
> >>   		if(nsems > SEMMSL_FAST) {
> >>   			if (!ipc_rcu_getref(sma)) {
> >> -				sem_unlock(sma, -1);
> >> -				rcu_read_unlock();
> >>   				err = -EIDRM;
> >> -				goto out_free;
> >> +				goto out_unlock;
> >>   			}
> >>   			sem_unlock(sma, -1);
> >>   			rcu_read_unlock();
> >> @@ -1301,10 +1305,13 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
> >>   			rcu_read_lock();
> >>   			sem_lock_and_putref(sma);
> >>   			if (sma->sem_perm.deleted) {
> >> -				sem_unlock(sma, -1);
> >> -				rcu_read_unlock();
> >>   				err = -EIDRM;
> ^^^^^^^^^^^^^^^^^^ check if nsems > SEMMSL_FAST
> >> -				goto out_free;
> >> +				goto out_unlock;
> >> +			}
> >> +		} else {
> >> +			if (sma->sem_perm.deleted) {
> >> +				err = -EIDRM;
> >> +				goto out_unlock;
> >>   			}
> > I'm a bit lost here. Why should we only check the existence of the sem
> > if nsems <= SEMMSL_FAST? Shouldn't the same should apply either way?
> It is checked in both branches:
> - the check for "nsems > SEMMSL_FAST" was always there, due to the 
> kmalloc, the lock is dropped.

Right but the same 'race with rmid' logic could apply independently of
nsems: between looking up the sma, doing the checks and taking the lock,
it could have been removed underneath us, so we should check for its
existence right after taking the lock, something like this instead:

 	case GETALL:
	{
		ushort __user *array = p;
		int i;

		sem_lock(sma, NULL, -1);

		if (sma->sem_perm.deleted) {
			sem_unlock(sma, -1);
			rcu_read_unlock();
			return -EIDRM;
		}

In this GETALL case we drop the lock later down the road for
ipc_alloc(), and then check for sem_perm.deleted right after we take it
again. That seems fine, but doesn't take away the fact of the race
mentioned above.

Thanks,
Davidlohr


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