APPLIED(X,B,D,E): [SRU][X/B/E/F][PATCH] Revert "ipc, sem: remove uneeded sem_undo_list lock usage in exit_sem()"
Khaled Elmously
khalid.elmously at canonical.com
Wed Mar 11 05:58:57 UTC 2020
On 2020-02-24 14:06:44 , Ioanna Alifieraki wrote:
> BugLink: https://bugs.launchpad.net/bugs/1858834
>
> 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@canonical.com
> Fixes: a97955844807 ("ipc,sem: remove uneeded sem_undo_list lock usage in exit_sem()")
> Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki at canonical.com>
> Acked-by: Manfred Spraul <manfred at colorfullife.com>
> Acked-by: Herton R. Krzesinski <herton at redhat.com>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: <malat at debian.org>
> Cc: Joel Fernandes (Google) <joel at joelfernandes.org>
> Cc: Davidlohr Bueso <dave at stgolabs.net>
> Cc: Jay Vosburgh <jay.vosburgh at canonical.com>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (cherry picked from commit edf28f4061afe4c2d9eb1c3323d90e882c1d6800)
> ---
> 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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