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>

4 files changed, 41 insertions, 19 deletions

diff --git a/ipc/msg.c b/ipc/msg.c
index 558aa91186b6..8983ea57d970 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -696,7 +696,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
                         goto out_unlock0;
 
                 /* raced with RMID? */
-                if (msq->q_perm.deleted) {
+                if (!ipc_valid_object(&msq->q_perm)) {
                         err = -EIDRM;
                         goto out_unlock0;
                 }
@@ -731,7 +731,8 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
                 ipc_lock_object(&msq->q_perm);
 
                 ipc_rcu_putref(msq, ipc_rcu_free);
-                if (msq->q_perm.deleted) {
+                /* raced with RMID? */
+                if (!ipc_valid_object(&msq->q_perm)) {
                         err = -EIDRM;
                         goto out_unlock0;
                 }
@@ -909,7 +910,7 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
                 ipc_lock_object(&msq->q_perm);
 
                 /* raced with RMID? */
-                if (msq->q_perm.deleted) {
+                if (!ipc_valid_object(&msq->q_perm)) {
                         msg = ERR_PTR(-EIDRM);
                         goto out_unlock0;
                 }
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;
diff --git a/ipc/shm.c b/ipc/shm.c
index 7a51443a51d6..1bc68f115842 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -975,6 +975,13 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
                         goto out_unlock1;
 
                 ipc_lock_object(&shp->shm_perm);
+
+                /* check if shm_destroy() is tearing down shp */
+                if (!ipc_valid_object(&shp->shm_perm)) {
+                        err = -EIDRM;
+                        goto out_unlock0;
+                }
+
                 if (!ns_capable(ns->user_ns, CAP_IPC_LOCK)) {
                         kuid_t euid = current_euid();
                         if (!uid_eq(euid, shp->shm_perm.uid) &&
@@ -989,13 +996,6 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
                 }
 
                 shm_file = shp->shm_file;
-
-                /* check if shm_destroy() is tearing down shp */
-                if (shm_file == NULL) {
-                        err = -EIDRM;
-                        goto out_unlock0;
-                }
-
                 if (is_file_hugepages(shm_file))
                         goto out_unlock0;
 
@@ -1116,7 +1116,7 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
         ipc_lock_object(&shp->shm_perm);
 
         /* check if shm_destroy() is tearing down shp */
-        if (shp->shm_file == NULL) {
+        if (!ipc_valid_object(&shp->shm_perm)) {
                 ipc_unlock_object(&shp->shm_perm);
                 err = -EIDRM;
                 goto out_unlock;
diff --git a/ipc/util.h b/ipc/util.h
index 59d78aa94987..d05b7085a887 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -185,6 +185,19 @@ static inline void ipc_unlock(struct kern_ipc_perm *perm)
         rcu_read_unlock();
 }
 
+/*
+ * ipc_valid_object() - helper to sort out IPC_RMID races for codepaths
+ * where the respective ipc_ids.rwsem is not being held down.
+ * Checks whether the ipc object is still around or if it's gone already, as
+ * ipc_rmid() may have already freed the ID while the ipc lock was spinning.
+ * Needs to be called with kern_ipc_perm.lock held -- exception made for one
+ * checkpoint case at sys_semtimedop() as noted in code commentary.
+ */
+static inline bool ipc_valid_object(struct kern_ipc_perm *perm)
+{
+        return perm->deleted == 0;
+}
+
 struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id);
 int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
                         struct ipc_ops *ops, struct ipc_params *params);