[PATCH] fix SEM_UNDO with namespaces, take 2

[Manfred Spraul <manfred@colorfullife.com>]

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Hi,

below is the second attempt to fix SEM_UNDO + unshare():
lookup_undo (in ipc/sem.c) is not namespace-aware, thus all entries in 
sysvsem.undo_list must be from the same namespace.
The patch enforces that by detaching the current thread from 
sysvsem.undo_list in switch_task_namespaces() if the ipc namespace is 
changed.

The patch boots and passes simple sysvsem+unshare tests.

Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>

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diff --git a/ipc/sem.c b/ipc/sem.c
index 0b45a4d..35841bd 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1298,6 +1298,7 @@ void exit_sem(struct task_struct *tsk)
 	undo_list = tsk->sysvsem.undo_list;
 	if (!undo_list)
 		return;
+	tsk->sysvsem.undo_list = NULL;
 
 	if (!atomic_dec_and_test(&undo_list->refcnt))
 		return;
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index f5d332c..ddeb9d1 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -211,6 +211,18 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 
 	might_sleep();
 
+	if ((p->nsproxy == NULL && new != NULL) ||
+		(p->nsproxy != NULL && new == NULL) ||
+		(p->nsproxy != NULL && new != NULL && p->nsproxy->ipc_ns != new->ipc_ns)) {
+		/* switching the IPC namespace is considered equivalent to sys_exit() wrt.
+		 * to outstanding SEM_UNDO undos: After switching to the new IPC namespace,
+		 * the semaphore arrays from the old namespace are not accessible anymore.
+		 * 
+		 * Additionally, an implicit sys_unshare(CLONE_SYSVSEM) is performed.
+		 */
+		exit_sem(p);
+	}
+
 	ns = p->nsproxy;
 
 	rcu_assign_pointer(p->nsproxy, new);