The patch titled Subject: Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()" has been added to the -mm tree. Its filename is revert-ipcsem-remove-uneeded-sem_undo_list-lock-usage-in-exit_sem.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/revert-ipcsem-remove-uneeded-sem_un... and later at http://ozlabs.org/~akpm/mmotm/broken-out/revert-ipcsem-remove-uneeded-sem_un...

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------------------------------------------------------ From: Ioanna Alifieraki ioanna-maria.alifieraki@canonical.com Subject: 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->list_proc.next 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->list_proc.next 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] https://bugzilla.redhat.com/show_bug.cgi?id=1694779

Link: http://lkml.kernel.org/r/20191211191318.11860-1-ioanna-maria.alifieraki@cano... Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()") Signed-off-by: Ioanna Alifieraki ioanna-maria.alifieraki@canonical.com Cc: Manfred Spraul manfred@colorfullife.com Cc: Herton Krzesinski herton@redhat.com Cc: Arnd Bergmann arnd@arndb.de Cc: Catalin Marinas catalin.marinas@arm.com Cc: malat@debian.org Cc: Joel Fernandes (Google) joel@joelfernandes.org Cc: Davidlohr Bueso dave@stgolabs.net Cc: Jay Vosburgh jay.vosburgh@canonical.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

ipc/sem.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/ipc/sem.c~revert-ipcsem-remove-uneeded-sem_undo_list-lock-usage-in-exit_sem +++ a/ipc/sem.c @@ -2368,11 +2368,9 @@ void exit_sem(struct task_struct *tsk) ipc_assert_locked_object(&sma->sem_perm); list_del(&un->list_id);

- /* 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); list_del_rcu(&un->list_proc); + spin_unlock(&ulp->lock);

/* perform adjustments registered in un */ for (i = 0; i < sma->sem_nsems; i++) { _

Patches currently in -mm which might be from ioanna-maria.alifieraki@canonical.com are

revert-ipcsem-remove-uneeded-sem_undo_list-lock-usage-in-exit_sem.patch