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SubjectRe: [PATCH 4/4] ipc/sem.c: rewrite undo list locking
Manfred Spraul wrote:
> The attached patch:
> - reverses the locking order of ulp->lock and sem_lock:
> Previously, it was first ulp->lock, then inside sem_lock.
> Now it's the other way around.
> - converts the undo structure to rcu.
>
> Benefits:
> - With the old locking order, IPC_RMID could not kfree the undo structures.
> The stale entries remained in the linked lists and were released later.
> - The patch fixes a a race in semtimedop(): if both IPC_RMID and a semget() that
> recreates exactly the same id happen between find_alloc_undo() and sem_lock,
> then semtimedop() would access already kfree'd memory.
>
> Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>

Reviewed-by: Nadia Derbey <Nadia.Derbey@bull.net>

2 comments embedded.


> ---
> include/linux/sem.h | 6 ++-
> ipc/sem.c | 145 ++++++++++++++++++++++++++++++++-------------------
> 2 files changed, 96 insertions(+), 55 deletions(-)
>
> diff --git a/include/linux/sem.h b/include/linux/sem.h
> index d425993..1b191c1 100644
> --- a/include/linux/sem.h
> +++ b/include/linux/sem.h
> @@ -78,6 +78,7 @@ struct seminfo {
>
> #ifdef __KERNEL__
> #include <asm/atomic.h>
> +#include <linux/rcupdate.h>
>
> struct task_struct;
>
> @@ -114,7 +115,10 @@ struct sem_queue {
> * when the process exits.
> */
> struct sem_undo {
> - struct list_head list_proc; /* per-process list: all undos from one process */
> + struct list_head list_proc; /* per-process list: all undos from one process. */
> + /* rcu protected */
> + struct rcu_head rcu; /* rcu struct for sem_undo() */
> + struct sem_undo_list *ulp; /* sem_undo_list for the process */
> struct list_head list_id; /* per semaphore array list: all undos for one array */
> int semid; /* semaphore set identifier */
> short * semadj; /* array of adjustments, one per semaphore */
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 38996c0..d0b2217 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -502,27 +502,35 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum)
> return semzcnt;
> }
>
> +void free_un(struct rcu_head *head)
> +{
> + struct sem_undo *un = container_of(head, struct sem_undo, rcu);
> + kfree(un);
> +}
> +
> /* Free a semaphore set. freeary() is called with sem_ids.rw_mutex locked
> * as a writer and the spinlock for this semaphore set hold. sem_ids.rw_mutex
> * remains locked on exit.
> */
> static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
> {
> - struct sem_undo *un;
> - struct sem_queue *q, *t;
> + struct sem_undo *un, *tu;
> + struct sem_queue *q, *tq;
> struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm);
>
> - /* Invalidate the existing undo structures for this semaphore set.
> - * (They will be freed without any further action in exit_sem()
> - * or during the next semop.)
> - */
> + /* Free the existing undo structures for this semaphore set. */
> assert_spin_locked(&sma->sem_perm.lock);
> - list_for_each_entry(un, &sma->list_id, list_id)
> + list_for_each_entry_safe(un, tu, &sma->list_id, list_id) {
> + list_del(&un->list_id);
> + spin_lock(&un->ulp->lock);
> un->semid = -1;
> + list_del_rcu(&un->list_proc);
> + spin_unlock(&un->ulp->lock);
> + call_rcu(&un->rcu, free_un);
> + }
>
> /* Wake up all pending processes and let them fail with EIDRM. */
> -
> - list_for_each_entry_safe(q, t, &sma->sem_pending, list) {
> + list_for_each_entry_safe(q, tq, &sma->sem_pending, list) {
> list_del(&q->list);
>
> q->status = IN_WAKEUP;
> @@ -946,16 +954,11 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp)
>
> static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
> {
> - struct sem_undo *walk, *tmp;
> + struct sem_undo *walk;
>
> - assert_spin_locked(&ulp->lock);
> - list_for_each_entry_safe(walk, tmp, &ulp->list_proc, list_proc) {
> + list_for_each_entry_rcu(walk, &ulp->list_proc, list_proc) {
> if(walk->semid==semid)
> return walk;
> - if(walk->semid==-1) {
> - list_del(&walk->list_proc);
> - kfree(walk);
> - }
> }
> return NULL;
> }
> @@ -968,6 +971,8 @@ static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
> * The function looks up (and if not present creates) the undo structure.
> * The size of the undo structure depends on the size of the semaphore
> * array, thus the alloc path is not that straightforward.
> + * Lifetime-rules: sem_undo is rcu-protected, on success, the function
> + * performs a rcu_read_lock().
> */
> static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
> {
> @@ -981,11 +986,13 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
> if (error)
> return ERR_PTR(error);
>
> + rcu_read_lock();
> spin_lock(&ulp->lock);
> un = lookup_undo(ulp, semid);
> spin_unlock(&ulp->lock);

Why are we locking the sem_undo_list: in the lookup, we are traversing
the proc_list that is rcu_protected.

> if (likely(un!=NULL))
> goto out;
> + rcu_read_unlock();
>
> /* no undo structure around - allocate one. */
> /* step 1: figure out the size of the semaphore array */
> @@ -1003,38 +1010,36 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
> return ERR_PTR(-ENOMEM);
> }
>
> - /* step 3: Acquire the lock on the undo list pointer */
> - spin_lock(&ulp->lock);
> -
> - /* step 4: check for races: someone else allocated the undo struct,
> - * semaphore array was destroyed.
> - */
> - un = lookup_undo(ulp, semid);
> - if (un) {
> - spin_unlock(&ulp->lock);
> - kfree(new);
> - sem_putref(sma);
> - goto out;
> - }
> + /* step 3: Acquire the lock on semaphore array */
> sem_lock_and_putref(sma);
> if (sma->sem_perm.deleted) {
> sem_unlock(sma);
> - spin_unlock(&ulp->lock);
> kfree(new);
> un = ERR_PTR(-EIDRM);
> goto out;
> }
> + spin_lock(&ulp->lock);
> +
> + /* step 4: check for races: did someone else allocate the undo struct? */
> + un = lookup_undo(ulp, semid);
> + if (un) {
> + kfree(new);
> + goto success;
> + }
> /* step 5: initialize & link new undo structure */
> new->semadj = (short *) &new[1];
> + new->ulp = ulp;
> new->semid = semid;
> assert_spin_locked(&ulp->lock);
> - list_add(&new->list_proc, &ulp->list_proc);
> + list_add_rcu(&new->list_proc, &ulp->list_proc);
> assert_spin_locked(&sma->sem_perm.lock);
> list_add(&new->list_id, &sma->list_id);
> + un = new;
>
> - sem_unlock(sma);
> +success:
> spin_unlock(&ulp->lock);
> - un = new;
> + rcu_read_lock();

Oh, I'm realizing that we should leave the routine with an rcu_read_lock?
Why not adding a comment everywhere find_alloc_undo() is called?

> + sem_unlock(sma);
> out:
> return un;
> }
> @@ -1101,6 +1106,8 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
>
> sma = sem_lock_check(ns, semid);
> if (IS_ERR(sma)) {
> + if (un)
> + rcu_read_unlock();
> error = PTR_ERR(sma);
> goto out_free;
> }
> @@ -1109,10 +1116,26 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops,
> * semid identifiers are not unique - find_alloc_undo may have
> * allocated an undo structure, it was invalidated by an RMID
> * and now a new array with received the same id. Check and fail.
> + * This case can be detected checking un->semid. The existance of
> + * "un" itself is guaranteed by rcu.
> */
> error = -EIDRM;
> - if (un && un->semid == -1)
> - goto out_unlock_free;
> + if (un) {
> + if(un->semid == -1) {
> + rcu_read_unlock();
> + goto out_unlock_free;
> + } else {
> + /*
> + * rcu lock can be released, "un" cannot disappear:
> + * - sem_lock is acquired, thus IPC_RMID is
> + * impossible.
> + * - exit_sem is impossible, it always operates on
> + * current (or a dead task).
> + */
> +
> + rcu_read_unlock();
> + }
> + }
>
> error = -EFBIG;
> if (max >= sma->sem_nsems)
> @@ -1240,7 +1263,6 @@ int copy_semundo(unsigned long clone_flags, struct task_struct *tsk)
> void exit_sem(struct task_struct *tsk)
> {
> struct sem_undo_list *ulp;
> - struct sem_undo *un, *tmp;
>
> ulp= tsk->sysvsem.undo_list;
> if (!ulp)
> @@ -1250,28 +1272,47 @@ void exit_sem(struct task_struct *tsk)
> if (!atomic_dec_and_test(&ulp->refcnt))
> return;
>
> - spin_lock(&ulp->lock);
> -
> - list_for_each_entry_safe(un, tmp, &ulp->list_proc, list_proc) {
> + for (;;) {
> struct sem_array *sma;
> + struct sem_undo *un;
> + int semid;
> int i;
>
> - if(un->semid == -1)
> - goto free;
> + rcu_read_lock();
> + un =list_entry(rcu_dereference(ulp->list_proc.next),
> + struct sem_undo, list_proc);
> + if (&un->list_proc == &ulp->list_proc)
> + semid = -1;
> + else
> + semid = un->semid;
> + rcu_read_unlock();
>
> - sma = sem_lock(tsk->nsproxy->ipc_ns, un->semid);
> - if (IS_ERR(sma))
> - goto free;
> + if(semid == -1)
> + break;
>
> - if (un->semid == -1)
> - goto unlock_free;
> + sma = sem_lock_check(tsk->nsproxy->ipc_ns, un->semid);
>
> - BUG_ON(sem_checkid(sma, un->semid));
> + /* exit_sem raced with IPC_RMID, nothing to do */
> + if (IS_ERR(sma))
> + continue;
> +
> + un = lookup_undo(ulp, semid);
> + if (un == NULL) {
> + /* exit_sem raced with IPC_RMID+semget() that created
> + * exactly the same semid. Nothing to do.
> + */
> + sem_unlock(sma);
> + continue;
> + }
>
> - /* remove un from sma->list_id */
> + /* remove un from the linked lists */
> assert_spin_locked(&sma->sem_perm.lock);
> list_del(&un->list_id);
>
> + 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++) {
> struct sem * semaphore = &sma->sem_base[i];
> @@ -1300,14 +1341,10 @@ void exit_sem(struct task_struct *tsk)
> sma->sem_otime = get_seconds();
> /* maybe some queued-up processes were waiting for this */
> update_queue(sma);
> -unlock_free:
> sem_unlock(sma);
> -free:
> - assert_spin_locked(&ulp->lock);
> - list_del(&un->list_proc);
> - kfree(un);
> +
> + call_rcu(&un->rcu, free_un);
> }
> - spin_unlock(&ulp->lock);
> kfree(ulp);
> }
>

Regards,
Nadia



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