path: root/ipc
diff options
authorIoanna Alifieraki <>2020-02-20 20:04:00 -0800
committerLinus Torvalds <>2020-02-21 11:22:15 -0800
commitedf28f4061afe4c2d9eb1c3323d90e882c1d6800 (patch)
tree55fe4c350dde81a7ce4cfc342ff3356fcea30bb7 /ipc
parentc766d1472c70d25ad475cf56042af1652e792b23 (diff)
Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()"
This reverts commit a97955844807e327df11aa33869009d14d6b7de0. Commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") removes a lock that is needed. This leads to a process looping infinitely in exit_sem() and can also lead to a crash. There is a reproducer available in [1] and with the commit reverted the issue does not reproduce anymore. Using the reproducer found in [1] is fairly easy to reach a point where one of the child processes is looping infinitely in exit_sem between for(;;) and if (semid == -1) block, while it's trying to free its last sem_undo structure which has already been freed by freeary(). Each sem_undo struct is on two lists: one per semaphore set (list_id) and one per process (list_proc). The list_id list tracks undos by semaphore set, and the list_proc by process. Undo structures are removed either by freeary() or by exit_sem(). The freeary function is invoked when the user invokes a syscall to remove a semaphore set. During this operation freeary() traverses the list_id associated with the semaphore set and removes the undo structures from both the list_id and list_proc lists. For this case, exit_sem() is called at process exit. Each process contains a struct sem_undo_list (referred to as "ulp") which contains the head for the list_proc list. When the process exits, exit_sem() traverses this list to remove each sem_undo struct. As in freeary(), whenever a sem_undo struct is removed from list_proc, it is also removed from the list_id list. Removing elements from list_id is safe for both exit_sem() and freeary() due to sem_lock(). Removing elements from list_proc is not safe; freeary() locks &un->ulp->lock when it performs list_del_rcu(&un->list_proc) but exit_sem() does not (locking was removed by commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()"). This can result in the following situation while executing the reproducer [1] : Consider a child process in exit_sem() and the parent in freeary() (because of semctl(sid[i], NSEM, IPC_RMID)). - The list_proc for the child contains the last two undo structs A and B (the rest have been removed either by exit_sem() or freeary()). - The semid for A is 1 and semid for B is 2. - exit_sem() removes A and at the same time freeary() removes B. - Since A and B have different semid sem_lock() will acquire different locks for each process and both can proceed. The bug is that they remove A and B from the same list_proc at the same time because only freeary() acquires the ulp lock. When exit_sem() removes A it makes ulp-> to point at B and at the same time freeary() removes B setting B->semid=-1. At the next iteration of for(;;) loop exit_sem() will try to remove B. The only way to break from for(;;) is for (&un->list_proc == &ulp->list_proc) to be true which is not. Then exit_sem() will check if B->semid=-1 which is and will continue looping in for(;;) until the memory for B is reallocated and the value at B->semid is changed. At that point, exit_sem() will crash attempting to unlink B from the lists (this can be easily triggered by running the reproducer [1] a second time). To prove this scenario instrumentation was added to keep information about each sem_undo (un) struct that is removed per process and per semaphore set (sma). CPU0 CPU1 [caller holds sem_lock(sma for A)] ... freeary() exit_sem() ... ... ... sem_lock(sma for B) spin_lock(A->ulp->lock) ... list_del_rcu(un_A->list_proc) list_del_rcu(un_B->list_proc) Undo structures A and B have different semid and sem_lock() operations proceed. However they belong to the same list_proc list and they are removed at the same time. This results into ulp-> pointing to the address of B which is already removed. After reverting commit a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") the issue was no longer reproducible. [1] Link: Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") Signed-off-by: Ioanna Alifieraki <> Acked-by: Manfred Spraul <> Acked-by: Herton R. Krzesinski <> Cc: Arnd Bergmann <> Cc: Catalin Marinas <> Cc: <> Cc: Joel Fernandes (Google) <> Cc: Davidlohr Bueso <> Cc: Jay Vosburgh <> Cc: <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'ipc')
1 files changed, 2 insertions, 4 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index 4f4303f32077..3687b71151b3 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -2384,11 +2384,9 @@ void exit_sem(struct task_struct *tsk)
- /* we are the last process using this ulp, acquiring ulp->lock
- * isn't required. Besides that, we are also protected against
- * IPC_RMID as we hold sma->sem_perm lock now
- */
+ spin_lock(&ulp->lock);
+ spin_unlock(&ulp->lock);
/* perform adjustments registered in un */
for (i = 0; i < sma->sem_nsems; i++) {