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Subject[PATCH] Revert "ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()"
Date
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

Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
---
ipc/sem.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index ec97a7072413..fe12ea8dd2b3 100644
--- a/ipc/sem.c
+++ b/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++) {
--
2.17.1
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