[PATCH 1/2] shm: skip shm_destroy if task IPC namespace was changed

[Alexander Mikhalitsyn]

Task may change IPC namespace by doing setns() but sysvshm
objects remains at the origin IPC namespace (=IPC namespace
where task was when shmget() was called). Let's skip forced
shm destroy in such case because we can't determine IPC
namespace by shm only. These problematic sysvshm's will
be destroyed on ipc namespace cleanup.

Fixes: ab602f79915 ("shm: make exit_shm work proportional to task activity")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Milton Miller <miltonm@xxxxxxx>
Cc: Jack Miller <millerjo@xxxxxxxxxx>
Cc: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
Cc: Alexander Mikhalitsyn <alexander@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn@xxxxxxxxxxxxx>
---
 ipc/shm.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 748933e376ca..70a41171b8bb 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -173,6 +173,14 @@ static inline struct shmid_kernel *shm_obtain_object_check(struct ipc_namespace
 	return container_of(ipcp, struct shmid_kernel, shm_perm);
 }
 
+static inline bool is_shm_in_ns(struct ipc_namespace *ns, struct shmid_kernel *shp)
+{
+	int idx = ipcid_to_idx(shp->shm_perm.id);
+	struct shmid_kernel *tshp = shm_obtain_object(ns, idx);
+
+	return !IS_ERR(tshp) && tshp == shp;
+}
+
 /*
  * shm_lock_(check_) routines are called in the paths where the rwsem
  * is not necessarily held.
@@ -415,7 +423,7 @@ void exit_shm(struct task_struct *task)
 	list_for_each_entry_safe(shp, n, &task->sysvshm.shm_clist, shm_clist) {
 		shp->shm_creator = NULL;
 
-		if (shm_may_destroy(ns, shp)) {
+		if (is_shm_in_ns(ns, shp) && shm_may_destroy(ns, shp)) {
 			shm_lock_by_ptr(shp);
 			shm_destroy(ns, shp);
 		}
-- 
2.31.1