Commit message: Initialize msg/shm IPC objects before doing ipc_addid()

As reported by Dmitry Vyukov, we really shouldn't do ipc_addid() before
having initialized the IPC object state.  Yes, we initialize the IPC
object in a locked state, but with all the lockless RCU lookup work,
that IPC object lock no longer means that the state cannot be seen.

We already did this for the IPC semaphore code (see commit e8577d1f0329:
"ipc/sem.c: fully initialize sem_array before making it visible") but we
clearly forgot about msg and shm.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2015-7613
CWE: CWE-362
Vulnerability description: Race condition in the IPC object implementation in the Linux kernel through 4.2.3 allows local users to gain privileges by triggering an ipc_addid call that leads to uid and gid comparisons against uninitialized data, related to msg.c, shm.c, and util.c.

Project: linux
File: msg.c

long do_msgsnd(int msqid, long mtype, void __user *mtext,
		size_t msgsz, int msgflg)
{
	struct msg_queue *msq;
	struct msg_msg *msg;
	int err;
	struct ipc_namespace *ns;

	ns = current->nsproxy->ipc_ns;

	if (msgsz > ns->msg_ctlmax || (long) msgsz < 0 || msqid < 0)
		return -EINVAL;
	if (mtype < 1)
		return -EINVAL;

	msg = load_msg(mtext, msgsz);
	if (IS_ERR(msg))
		return PTR_ERR(msg);

	msg->m_type = mtype;
	msg->m_ts = msgsz;

	rcu_read_lock();
	msq = msq_obtain_object_check(ns, msqid);
	if (IS_ERR(msq)) {
		err = PTR_ERR(msq);
		goto out_unlock1;
	}

	ipc_lock_object(&msq->q_perm);

	for (;;) {
		struct msg_sender s;

		err = -EACCES;
		if (ipcperms(ns, &msq->q_perm, S_IWUGO))
			goto out_unlock0;

		/* raced with RMID? */
		if (!ipc_valid_object(&msq->q_perm)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		err = security_msg_queue_msgsnd(msq, msg, msgflg);
		if (err)
			goto out_unlock0;

		if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
				1 + msq->q_qnum <= msq->q_qbytes) {
			break;
		}

		/* queue full, wait: */
		if (msgflg & IPC_NOWAIT) {
			err = -EAGAIN;
			goto out_unlock0;
		}

		/* enqueue the sender and prepare to block */
		ss_add(msq, &s);

		if (!ipc_rcu_getref(msq)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		ipc_unlock_object(&msq->q_perm);
		rcu_read_unlock();
		schedule();

		rcu_read_lock();
		ipc_lock_object(&msq->q_perm);

		ipc_rcu_putref(msq, ipc_rcu_free);
		/* raced with RMID? */
		if (!ipc_valid_object(&msq->q_perm)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		ss_del(&s);

		if (signal_pending(current)) {
			err = -ERESTARTNOHAND;
			goto out_unlock0;
		}

	}
	msq->q_lspid = task_tgid_vnr(current);
	msq->q_stime = get_seconds();

	if (!pipelined_send(msq, msg)) {
		/* no one is waiting for this message, enqueue it */
		list_add_tail(&msg->m_list, &msq->q_messages);
		msq->q_cbytes += msgsz;
		msq->q_qnum++;
		atomic_add(msgsz, &ns->msg_bytes);
		atomic_inc(&ns->msg_hdrs);
	}

	err = 0;
	msg = NULL;

out_unlock0:
	ipc_unlock_object(&msq->q_perm);
out_unlock1:
	rcu_read_unlock();
	if (msg != NULL)
		free_msg(msg);
	return err;
}

Project: linux
File: msg.c

static struct msg_msg *find_msg(struct msg_queue *msq, long *msgtyp, int mode)
{
	struct msg_msg *msg, *found = NULL;
	long count = 0;

	list_for_each_entry(msg, &msq->q_messages, m_list) {
		if (testmsg(msg, *msgtyp, mode) &&
		    !security_msg_queue_msgrcv(msq, msg, current,
					       *msgtyp, mode)) {
			if (mode == SEARCH_LESSEQUAL && msg->m_type != 1) {
				*msgtyp = msg->m_type - 1;
				found = msg;
			} else if (mode == SEARCH_NUMBER) {
				if (*msgtyp == count)
					return msg;
			} else
				return msg;
			count++;
		}
	}

	return found ?: ERR_PTR(-EAGAIN);
}

Project: linux
File: msg.c

void msg_init_ns(struct ipc_namespace *ns)
{
	ns->msg_ctlmax = MSGMAX;
	ns->msg_ctlmnb = MSGMNB;
	ns->msg_ctlmni = MSGMNI;

	atomic_set(&ns->msg_bytes, 0);
	atomic_set(&ns->msg_hdrs, 0);
	ipc_init_ids(&ns->ids[IPC_MSG_IDS]);
}

Project: linux
File: msg.c

static inline void ss_add(struct msg_queue *msq, struct msg_sender *mss)
{
	mss->tsk = current;
	__set_current_state(TASK_INTERRUPTIBLE);
	list_add_tail(&mss->list, &msq->q_senders);
}

Project: linux
File: shm.c

SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
{
	struct mm_struct *mm = current->mm;
	struct vm_area_struct *vma;
	unsigned long addr = (unsigned long)shmaddr;
	int retval = -EINVAL;
#ifdef CONFIG_MMU
	loff_t size = 0;
	struct file *file;
	struct vm_area_struct *next;
#endif

	if (addr & ~PAGE_MASK)
		return retval;

	down_write(&mm->mmap_sem);

	/*
	 * This function tries to be smart and unmap shm segments that
	 * were modified by partial mlock or munmap calls:
	 * - It first determines the size of the shm segment that should be
	 *   unmapped: It searches for a vma that is backed by shm and that
	 *   started at address shmaddr. It records it's size and then unmaps
	 *   it.
	 * - Then it unmaps all shm vmas that started at shmaddr and that
	 *   are within the initially determined size and that are from the
	 *   same shm segment from which we determined the size.
	 * Errors from do_munmap are ignored: the function only fails if
	 * it's called with invalid parameters or if it's called to unmap
	 * a part of a vma. Both calls in this function are for full vmas,
	 * the parameters are directly copied from the vma itself and always
	 * valid - therefore do_munmap cannot fail. (famous last words?)
	 */
	/*
	 * If it had been mremap()'d, the starting address would not
	 * match the usual checks anyway. So assume all vma's are
	 * above the starting address given.
	 */
	vma = find_vma(mm, addr);

#ifdef CONFIG_MMU
	while (vma) {
		next = vma->vm_next;

		/*
		 * Check if the starting address would match, i.e. it's
		 * a fragment created by mprotect() and/or munmap(), or it
		 * otherwise it starts at this address with no hassles.
		 */
		if ((vma->vm_ops == &shm_vm_ops) &&
			(vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) {

			/*
			 * Record the file of the shm segment being
			 * unmapped.  With mremap(), someone could place
			 * page from another segment but with equal offsets
			 * in the range we are unmapping.
			 */
			file = vma->vm_file;
			size = i_size_read(file_inode(vma->vm_file));
			do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
			/*
			 * We discovered the size of the shm segment, so
			 * break out of here and fall through to the next
			 * loop that uses the size information to stop
			 * searching for matching vma's.
			 */
			retval = 0;
			vma = next;
			break;
		}
		vma = next;
	}

	/*
	 * We need look no further than the maximum address a fragment
	 * could possibly have landed at. Also cast things to loff_t to
	 * prevent overflows and make comparisons vs. equal-width types.
	 */
	size = PAGE_ALIGN(size);
	while (vma && (loff_t)(vma->vm_end - addr) <= size) {
		next = vma->vm_next;

		/* finding a matching vma now does not alter retval */
		if ((vma->vm_ops == &shm_vm_ops) &&
		    ((vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) &&
		    (vma->vm_file == file))
			do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
		vma = next;
	}

#else /* CONFIG_MMU */
	/* under NOMMU conditions, the exact address to be destroyed must be
	 * given */
	if (vma && vma->vm_start == addr && vma->vm_ops == &shm_vm_ops) {
		do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
		retval = 0;
	}

#endif

	up_write(&mm->mmap_sem);
	return retval;
}

Project: linux
File: util.c

static int ipcget_new(struct ipc_namespace *ns, struct ipc_ids *ids,
		const struct ipc_ops *ops, struct ipc_params *params)
{
	int err;

	down_write(&ids->rwsem);
	err = ops->getnew(ns, params);
	up_write(&ids->rwsem);
	return err;
}