ipc: introduce ipc_valid_object() helper to sort out IPC_RMID races

After the locking semantics for the SysV IPC API got improved, a couple
of IPC_RMID race windows were opened because we ended up dropping the
'kern_ipc_perm.deleted' check performed way down in ipc_lock().  The
spotted races got sorted out by re-introducing the old test within the
racy critical sections.

This patch introduces ipc_valid_object() to consolidate the way we cope
with IPC_RMID races by using the same abstraction across the API
implementation.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 8 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index cc9ac35b793c..4d88194a5ffe 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1284,7 +1284,7 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
 
         sem_lock(sma, NULL, -1);
 
-        if (sma->sem_perm.deleted) {
+        if (!ipc_valid_object(&sma->sem_perm)) {
                 sem_unlock(sma, -1);
                 rcu_read_unlock();
                 return -EIDRM;
@@ -1344,7 +1344,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
                 int i;
 
                 sem_lock(sma, NULL, -1);
-                if (sma->sem_perm.deleted) {
+                if (!ipc_valid_object(&sma->sem_perm)) {
                         err = -EIDRM;
                         goto out_unlock;
                 }
@@ -1363,7 +1363,7 @@ 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) {
+                        if (!ipc_valid_object(&sma->sem_perm)) {
                                 err = -EIDRM;
                                 goto out_unlock;
                         }
@@ -1411,7 +1411,7 @@ 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) {
+                if (!ipc_valid_object(&sma->sem_perm)) {
                         err = -EIDRM;
                         goto out_unlock;
                 }
@@ -1437,7 +1437,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
                 goto out_rcu_wakeup;
 
         sem_lock(sma, NULL, -1);
-        if (sma->sem_perm.deleted) {
+        if (!ipc_valid_object(&sma->sem_perm)) {
                 err = -EIDRM;
                 goto out_unlock;
         }
@@ -1701,7 +1701,7 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
         /* step 3: Acquire the lock on semaphore array */
         rcu_read_lock();
         sem_lock_and_putref(sma);
-        if (sma->sem_perm.deleted) {
+        if (!ipc_valid_object(&sma->sem_perm)) {
                 sem_unlock(sma, -1);
                 rcu_read_unlock();
                 kfree(new);
@@ -1848,7 +1848,15 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 
         error = -EIDRM;
         locknum = sem_lock(sma, sops, nsops);
-        if (sma->sem_perm.deleted)
+        /*
+         * We eventually might perform the following check in a lockless
+         * fashion, considering ipc_valid_object() locking constraints.
+         * If nsops == 1 and there is no contention for sem_perm.lock, then
+         * only a per-semaphore lock is held and it's OK to proceed with the
+         * check below. More details on the fine grained locking scheme
+         * entangled here and why it's RMID race safe on comments at sem_lock()
+         */
+        if (!ipc_valid_object(&sma->sem_perm))
                 goto out_unlock_free;
         /*
          * semid identifiers are not unique - find_alloc_undo may have
@@ -2070,7 +2078,7 @@ void exit_sem(struct task_struct *tsk)
 
                 sem_lock(sma, NULL, -1);
                 /* exit_sem raced with IPC_RMID, nothing to do */
-                if (sma->sem_perm.deleted) {
+                if (!ipc_valid_object(&sma->sem_perm)) {
                         sem_unlock(sma, -1);
                         rcu_read_unlock();
                         continue;