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

[Manfred Spraul]

Andrew Morton wrote:
> On Sun, 13 Apr 2008 10:04:17 +0200 Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
>
>   
> > 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)
> >
> >     
>
> Is this a non-back-compatible change?
>
>   
It adds a new feature - previously sys_unshare(CLONE_SYSVSEM) returned 
-EINVAL.

> > Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> > ---
> >  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;
> > +	}
> >     
>
> And can we do anything nicer than this?
>
>   
Attached is an alternative. If you prefer it, I'll send another patch set.

--
   Manfred
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..535aa92 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1675,18 +1675,6 @@ 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
- */
-static int unshare_semundo(unsigned long unshare_flags, struct sem_undo_list **new_ulistp)
-{
-	if (unshare_flags & CLONE_SYSVSEM)
-		return -EINVAL;
-
-	return 0;
-}
-
-/*
  * unshare allows a process to 'unshare' part of the process
  * context which was originally shared using clone.  copy_*
  * functions used by do_fork() cannot be used here directly
@@ -1701,7 +1689,6 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 	struct sighand_struct *new_sigh = NULL;
 	struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
 	struct files_struct *fd, *new_fd = NULL;
-	struct sem_undo_list *new_ulist = NULL;
 	struct nsproxy *new_nsproxy = NULL;
 
 	check_unshare_flags(&unshare_flags);
@@ -1724,13 +1711,17 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 		goto bad_unshare_cleanup_sigh;
 	if ((err = unshare_fd(unshare_flags, &new_fd)))
 		goto bad_unshare_cleanup_vm;
-	if ((err = unshare_semundo(unshare_flags, &new_ulist)))
-		goto bad_unshare_cleanup_fd;
 	if ((err = unshare_nsproxy_namespaces(unshare_flags, &new_nsproxy,
 			new_fs)))
-		goto bad_unshare_cleanup_semundo;
+		goto bad_unshare_cleanup_fd;
 
-	if (new_fs ||  new_mm || new_fd || new_ulist || new_nsproxy) {
+	if (new_fs ||  new_mm || new_fd || (unshare_flags & CLONE_SYSVSEM) || 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);
@@ -1766,7 +1757,6 @@ asmlinkage long sys_unshare(unsigned long unshare_flags)
 	if (new_nsproxy)
 		put_nsproxy(new_nsproxy);
 
-bad_unshare_cleanup_semundo:
 bad_unshare_cleanup_fd:
 	if (new_fd)
 		put_files_struct(new_fd);