[kernel-hardening] [PATCH v2] shm: optimize locking and ipc_namespace getting

Vasiliy Kulikov
2011-07-05T17:38:51Z

shm_lock() does a lookup of shm segment in shm_ids(ns).ipcs_idr, which
is redundant as we already know shmid_kernel address. An actual lock is
also not required for reads until we really want to destroy the segment.

exit_shm() and shm_destroy_orphaned() may avoid the loop by checking
whether there is at least one segment in current ipc_namespace.

The check of nsproxy and ipc_ns against NULL is redundant as exit_shm()
is called from do_exit() before the call to exit_notify(), so the
dereferencing current->nsproxy->ipc_ns is guaranteed to be safe.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
---
ipc/shm.c | 61 ++++++++++++++++++++++++++++---------------------------------
1 files changed, 28 insertions(+), 33 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 3baae98..85db561 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -131,6 +131,12 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
return container_of(ipcp, struct shmid_kernel, shm_perm);
}

+static inline void shm_lock_by_ptr(struct shmid_kernel *ipcp)
+{
+ rcu_read_lock();
+ spin_lock(&ipcp->shm_perm.lock);
+}
+
static inline struct shmid_kernel *shm_lock_check(struct ipc_namespace *ns,
int id)
{
@@ -231,18 +237,15 @@ static void shm_close(struct vm_area_struct *vma)
up_write(&shm_ids(ns).rw_mutex);
}

+/* Called with ns->shm_ids(ns).rw_mutex locked */
static int shm_try_destroy_current(int id, void *p, void *data)
{
struct ipc_namespace *ns = data;
- struct shmid_kernel *shp = shm_lock(ns, id);
+ struct kern_ipc_perm *ipcp = p;
+ struct shmid_kernel *shp = container_of(ipcp, struct shmid_kernel, shm_perm);

- if (IS_ERR(shp))
- return 0;
-
- if (shp->shm_creator != current) {
- shm_unlock(shp);
+ if (shp->shm_creator != current)
return 0;
- }

/*
* Mark it as orphaned to destroy the segment when
@@ -255,64 +258,56 @@ static int shm_try_destroy_current(int id, void *p, void *data)
* Don't even try to destroy it. If shm_rmid_forced=0 and IPC_RMID
* is not set, it shouldn't be deleted here.
*/
- if (!ns->shm_rmid_forced) {
- shm_unlock(shp);
+ if (!ns->shm_rmid_forced)
return 0;
- }

- if (shm_may_destroy(ns, shp))
+ if (shm_may_destroy(ns, shp)) {
+ shm_lock_by_ptr(shp);
shm_destroy(ns, shp);
- else
- shm_unlock(shp);
+ }
return 0;
}

+/* Called with ns->shm_ids(ns).rw_mutex locked */
static int shm_try_destroy_orphaned(int id, void *p, void *data)
{
struct ipc_namespace *ns = data;
- struct shmid_kernel *shp = shm_lock(ns, id);
-
- if (IS_ERR(shp))
- return 0;
+ struct kern_ipc_perm *ipcp = p;
+ struct shmid_kernel *shp = container_of(ipcp, struct shmid_kernel, shm_perm);

/*
* We want to destroy segments without users and with already
* exit'ed originating process.
+ *
+ * As shp->* are changed under rw_mutex, it's safe to skip shp locking.
*/
- if (shp->shm_creator != NULL) {
- shm_unlock(shp);
+ if (shp->shm_creator != NULL)
return 0;
- }

- if (shm_may_destroy(ns, shp))
+ if (shm_may_destroy(ns, shp)) {
+ shm_lock_by_ptr(shp);
shm_destroy(ns, shp);
- else
- shm_unlock(shp);
+ }
return 0;
}

void shm_destroy_orphaned(struct ipc_namespace *ns)
{
down_write(&shm_ids(ns).rw_mutex);
- idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_orphaned, ns);
+ if (&shm_ids(ns).in_use)
+ idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_orphaned, ns);
up_write(&shm_ids(ns).rw_mutex);
}

void exit_shm(struct task_struct *task)
{
- struct nsproxy *nsp = task->nsproxy;
- struct ipc_namespace *ns;
-
- if (!nsp)
- return;
- ns = nsp->ipc_ns;
- if (!ns)
- return;
+ struct ipc_namespace *ns = task->nsproxy->ipc_ns;

/* Destroy all already created segments, but not mapped yet */
down_write(&shm_ids(ns).rw_mutex);
- idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_current, ns);
+ if (&shm_ids(ns).in_use)
+ idr_for_each(&shm_ids(ns).ipcs_idr, &shm_try_destroy_current, ns);
up_write(&shm_ids(ns).rw_mutex);
}

--
1.7.0.4