From: Ioanna Alifieraki <ioanna-maria.alifieraki@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx
Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@xxxxxxxxxxxxx>
Acked-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Acked-by: Herton R. Krzesinski <herton@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: <malat@xxxxxxxxxx>
Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Jay Vosburgh <jay.vosburgh@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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
@@ -2384,11 +2384,9 @@ 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++) {
_
