[RFC] shm: extend forced shm destroy to support objects from several IPC nses (simplified)

From: Eric W. Biederman
Date: Fri Nov 05 2021 - 17:35:26 EST


I have to dash so this is short.

This is what I am thinking this change should look like.

I am not certain this is truly reviewable as a single change, so I will
break it into a couple of smaller ones next time I get the chance.

Eric

include/linux/ipc_namespace.h | 12 ++++
include/linux/sched/task.h | 2 +-
ipc/shm.c | 135 +++++++++++++++++++++++++-----------------
ipc/util.c | 5 ++
ipc/util.h | 1 +
kernel/fork.c | 1 -
6 files changed, 100 insertions(+), 56 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 05e22770af51..c220767a0cc1 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -131,6 +131,13 @@ static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
return ns;
}

+static inline struct ipc_namespace *get_ipc_ns_not_zero(struct ipc_namespace *ns)
+{
+ if (ns && refcount_inc_not_zero(&ns->ns.count))
+ return ns;
+ return NULL;
+}
+
extern void put_ipc_ns(struct ipc_namespace *ns);
#else
static inline struct ipc_namespace *copy_ipcs(unsigned long flags,
@@ -147,6 +154,11 @@ static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
return ns;
}

+static inline struct ipc_namespace *get_ipc_ns_not_zero(struct ipc_namespace *ns)
+{
+ return ns;
+}
+
static inline void put_ipc_ns(struct ipc_namespace *ns)
{
}
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ef02be869cf2..1d9533d66f7e 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -157,7 +157,7 @@ static inline struct vm_struct *task_stack_vm_area(const struct task_struct *t)
* Protects ->fs, ->files, ->mm, ->group_info, ->comm, keyring
* subscriptions and synchronises with wait4(). Also used in procfs. Also
* pins the final release of task.io_context. Also protects ->cpuset and
- * ->cgroup.subsys[]. And ->vfork_done.
+ * ->cgroup.subsys[]. And ->vfork_done. And ->shmvshm.shm_clist.
*
* Nests both inside and outside of read_lock(&tasklist_lock).
* It must not be nested with write_lock_irq(&tasklist_lock),
diff --git a/ipc/shm.c b/ipc/shm.c
index ab749be6d8b7..80e3595d3a69 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -63,8 +63,9 @@ struct shmid_kernel /* private to the kernel */
struct ucounts *mlock_ucounts;

/* The task created the shm object. NULL if the task is dead. */
- struct task_struct *shm_creator;
+ struct task_struct __rcu *shm_creator;
struct list_head shm_clist; /* list by creator */
+ struct ipc_namespace *shm_ns; /* valid when shm_nattch != 0 */
} __randomize_layout;

/* shm_mode upper byte flags */
@@ -106,29 +107,17 @@ void shm_init_ns(struct ipc_namespace *ns)
ipc_init_ids(&shm_ids(ns));
}

-/*
- * Called with shm_ids.rwsem (writer) and the shp structure locked.
- * Only shm_ids.rwsem remains locked on exit.
- */
-static void do_shm_rmid(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
+static void do_shm_destroy(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
- struct shmid_kernel *shp;
-
- shp = container_of(ipcp, struct shmid_kernel, shm_perm);
-
- if (shp->shm_nattch) {
- shp->shm_perm.mode |= SHM_DEST;
- /* Do not find it any more */
- ipc_set_key_private(&shm_ids(ns), &shp->shm_perm);
- shm_unlock(shp);
- } else
- shm_destroy(ns, shp);
+ struct shmid_kernel *shp =
+ container_of(ipcp, struct shmid_kernel, shm_perm);
+ shm_destroy(ns, shp);
}

#ifdef CONFIG_IPC_NS
void shm_exit_ns(struct ipc_namespace *ns)
{
- free_ipcs(ns, &shm_ids(ns), do_shm_rmid);
+ free_ipcs(ns, &shm_ids(ns), do_shm_destroy);
idr_destroy(&ns->ids[IPC_SHM_IDS].ipcs_idr);
rhashtable_destroy(&ns->ids[IPC_SHM_IDS].key_ht);
}
@@ -225,9 +214,22 @@ static void shm_rcu_free(struct rcu_head *head)
kfree(shp);
}

+static inline void shm_clist_del(struct shmid_kernel *shp)
+{
+ struct task_struct *creator;
+
+ rcu_read_lock();
+ creator = rcu_dereference(shp->shm_creator);
+ if (creator) {
+ task_lock(creator);
+ list_del(&shp->shm_clist);
+ task_unlock(creator);
+ }
+ rcu_read_unlock();
+}
+
static inline void shm_rmid(struct ipc_namespace *ns, struct shmid_kernel *s)
{
- list_del(&s->shm_clist);
ipc_rmid(&shm_ids(ns), &s->shm_perm);
}

@@ -283,7 +285,9 @@ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
shm_file = shp->shm_file;
shp->shm_file = NULL;
ns->shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ shm_clist_del(shp);
shm_rmid(ns, shp);
+ shp->shm_ns = NULL;
shm_unlock(shp);
if (!is_file_hugepages(shm_file))
shmem_lock(shm_file, 0, shp->mlock_ucounts);
@@ -361,7 +365,7 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
*
* As shp->* are changed under rwsem, it's safe to skip shp locking.
*/
- if (shp->shm_creator != NULL)
+ if (rcu_access_pointer(shp->shm_creator) != NULL)
return 0;

if (shm_may_destroy(ns, shp)) {
@@ -382,48 +386,62 @@ void shm_destroy_orphaned(struct ipc_namespace *ns)
/* Locking assumes this will only be called with task == current */
void exit_shm(struct task_struct *task)
{
- struct ipc_namespace *ns = task->nsproxy->ipc_ns;
- struct shmid_kernel *shp, *n;
-
- if (list_empty(&task->sysvshm.shm_clist))
- return;
-
- /*
- * If kernel.shm_rmid_forced is not set then only keep track of
- * which shmids are orphaned, so that a later set of the sysctl
- * can clean them up.
- */
- if (!ns->shm_rmid_forced) {
- down_read(&shm_ids(ns).rwsem);
- list_for_each_entry(shp, &task->sysvshm.shm_clist, shm_clist)
- shp->shm_creator = NULL;
- /*
- * Only under read lock but we are only called on current
- * so no entry on the list will be shared.
- */
- list_del(&task->sysvshm.shm_clist);
- up_read(&shm_ids(ns).rwsem);
- return;
- }
+ struct list_head *head = &task->sysvshm.shm_clist;

/*
* Destroy all already created segments, that were not yet mapped,
* and mark any mapped as orphan to cover the sysctl toggling.
* Destroy is skipped if shm_may_destroy() returns false.
*/
- down_write(&shm_ids(ns).rwsem);
- list_for_each_entry_safe(shp, n, &task->sysvshm.shm_clist, shm_clist) {
- shp->shm_creator = NULL;
+ for (;;) {
+ struct ipc_namespace *ns;
+ struct shmid_kernel *shp;

- if (shm_may_destroy(ns, shp)) {
+ task_lock(task);
+ if (list_empty(head)) {
+ task_unlock(task);
+ break;
+ }
+
+ shp = list_first_entry(head, struct shmid_kernel, shm_clist);
+
+ list_del(&shp->shm_clist);
+ rcu_assign_pointer(shp->shm_creator, NULL);
+
+ /*
+ * Guarantee that ns lives after task_list is dropped.
+ *
+ * This shm segment may not be attached and it's ipc
+ * namespace may be exiting. If so ignore the shm
+ * segment as it will be destroyed by shm_exit_ns.
+ */
+ ns = get_ipc_ns_not_zero(shp->shm_ns);
+ if (!ns) {
+ task_unlock(task);
+ continue;
+ }
+
+ /* Guarantee shp lives after task_lock is dropped */
+ ipc_getref(&shp->shm_perm);
+
+ /* Drop task_lock so that shm_destroy may take it */
+ task_unlock(task);
+
+ /* Can the shm segment be destroyed? */
+ down_write(&shm_ids(ns).rwsem);
+ shm_lock_by_ptr(shp);
+ if (ipc_valid_object(&shp->shm_perm) &&
+ shm_may_destroy(ns, shp)) {
shm_lock_by_ptr(shp);
shm_destroy(ns, shp);
+ } else {
+ shm_unlock(shp);
}
- }

- /* Remove the list head from any segments still attached. */
- list_del(&task->sysvshm.shm_clist);
- up_write(&shm_ids(ns).rwsem);
+ ipc_rcu_putref(&shp->shm_perm, shm_rcu_free);
+ up_write(&shm_ids(ns).rwsem);
+ put_ipc_ns(ns);
+ }
}

static vm_fault_t shm_fault(struct vm_fault *vmf)
@@ -673,14 +691,17 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
shp->shm_segsz = size;
shp->shm_nattch = 0;
shp->shm_file = file;
- shp->shm_creator = current;
+ RCU_INIT_POINTER(shp->shm_creator, current);
+ shp->shm_ns = ns;

/* ipc_addid() locks shp upon success. */
error = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
if (error < 0)
goto no_id;

+ task_lock(current);
list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
+ task_unlock(current);

/*
* shmid gets reported as "inode#" in /proc/pid/maps.
@@ -913,8 +934,14 @@ static int shmctl_down(struct ipc_namespace *ns, int shmid, int cmd,
switch (cmd) {
case IPC_RMID:
ipc_lock_object(&shp->shm_perm);
- /* do_shm_rmid unlocks the ipc object and rcu */
- do_shm_rmid(ns, ipcp);
+ if (shp->shm_nattch) {
+ shp->shm_perm.mode |= SHM_DEST;
+ /* Do not find it any more */
+ ipc_set_key_private(&shm_ids(ns), &shp->shm_perm);
+ shm_unlock(shp);
+ } else
+ shm_destroy(ns, shp);
+ /* shm_unlock unlocked the ipc object and rcu */
goto out_up;
case IPC_SET:
ipc_lock_object(&shp->shm_perm);
diff --git a/ipc/util.c b/ipc/util.c
index fa2d86ef3fb8..58228f342397 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -525,6 +525,11 @@ void ipc_set_key_private(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
ipcp->key = IPC_PRIVATE;
}

+void ipc_getref(struct kern_ipc_perm *ptr)
+{
+ return refcount_inc(&ptr->refcount);
+}
+
bool ipc_rcu_getref(struct kern_ipc_perm *ptr)
{
return refcount_inc_not_zero(&ptr->refcount);
diff --git a/ipc/util.h b/ipc/util.h
index 2dd7ce0416d8..e13b46ff675f 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -170,6 +170,7 @@ static inline int ipc_get_maxidx(struct ipc_ids *ids)
* refcount is initialized by ipc_addid(), before that point call_rcu()
* must be used.
*/
+void ipc_getref(struct kern_ipc_perm *ptr);
bool ipc_rcu_getref(struct kern_ipc_perm *ptr);
void ipc_rcu_putref(struct kern_ipc_perm *ptr,
void (*func)(struct rcu_head *head));
diff --git a/kernel/fork.c b/kernel/fork.c
index 38681ad44c76..3e881f78bcf2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3095,7 +3095,6 @@ int ksys_unshare(unsigned long unshare_flags)
if (unshare_flags & CLONE_NEWIPC) {
/* Orphan segments in old ns (see sem above). */
exit_shm(current);
- shm_init_task(current);
}

if (new_nsproxy)