[PATCH 1/2] fix sys_unshare()+SEM_UNDO: add support for CLONE_SYSVSEM

[Manfred Spraul <manfred@colorfullife.com>]

sys_unshare(CLONE_NEWIPC) doesn't handle the undo lists properly, this can
cause a kernel memory corruption. CLONE_NEWIPC must detach from the existing
undo lists.
Fix, part 1: add support for sys_unshare(CLONE_SYSVSEM)

I'm not sure who's the right maintainer, Andrew, could you merge it?

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
---
 ipc/sem.c     |    1 +
 kernel/fork.c |   18 ++++++++++++++----
 2 files changed, 15 insertions(+), 4 deletions(-)

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/fork.c b/kernel/fork.c
index 9c042f9..7f242b0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1675,13 +1675,17 @@ static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp
 }
 
 /*
- * Unsharing of semundo for tasks created with CLONE_SYSVSEM is not
- * supported yet
+ * Unsharing of semundo for tasks created with CLONE_SYSVSEM doesn't require
+ * any allocations: it means that the task leaves the existing undo lists,
+ * just like sys_exit(). The new undo lists are allocated on demand in the
+ * ipc syscalls.
+ * new_ulistp is set to a non-NULL value, the caller expects that on success.
  */
 static int unshare_semundo(unsigned long unshare_flags, struct sem_undo_list **new_ulistp)
 {
-	if (unshare_flags & CLONE_SYSVSEM)
-		return -EINVAL;
+	if (unshare_flags & CLONE_SYSVSEM) {
+		*new_ulistp = (void*)1;
+	}
 
 	return 0;
 }
@@ -1731,6 +1735,12 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 		goto bad_unshare_cleanup_semundo;
 
 	if (new_fs ||  new_mm || new_fd || new_ulist || new_nsproxy) {
+		if (unshare_flags & CLONE_SYSVSEM) {
+			/*
+			 * CLONE_SYSVSEM is equivalent to sys_exit().
+			 */
+			exit_sem(current);
+		}
 
 		if (new_nsproxy) {
 			switch_task_namespaces(current, new_nsproxy);
-- 
1.5.4.1