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Date: Fri, 25 Jan 2019 10:46:58 -0500 (EST)
From: Vladis Dronov <vdronov@...hat.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2018-16880 Linux kernel: oob-write in
 drivers/vhost/net.c:get_rx_bufs()

Hello,

A flaw was found in the Linux kernel in the handle_rx() function in
the [vhost_net] driver. A malicious virtual guest under specific conditions can
trigger an out-of-bounds write in a kmalloc-8 slab on a virtual host which may
lead to a kernel memory corruption and a system panic. Due to the nature of
the flaw, privilege escalation cannot be fully ruled out, although we believe it
is unlikely.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1656472

An id CVE-2018-16880 was assigned to this flaw and we would like to ask to use
it in the related public communications.

This flaw was found and researched by Jason Wang, Red Hat Inc.

The suggested patch is below. Please, note, this is preliminary internal patch
which was not reviewed and accepted by the upstream community. The researcher
is going to send the patch to the related kernel mailing lists.

A reproducer can be provided via direct off-list request to me.

Please, see some details extracted from discussion with a researcher:

> is this guest triggerable (guest -> host) or host -> host?

a vm guest can trigger an oob-write on a host but requires a large network packet
to be received for this.

> what is overwritten?

kmalloc-8 slab on a vm host.

> what's the minimum and maximum size of the out-of-bound write?

from 8 bytes (sizeof vring_used_elem) to 504 bytes (63 * sizeof(vring_used_elem))

> does the attacker control the data that are written and if yes, to which degree?

attacker can not directly control the data.

Best regards,
Vladis Dronov | Red Hat, Inc. | Product Security | Senior Software Engineer

===[ 0001-vhost-fix-OOB-in-get_rx_bufs.patch ]===
From: Jason Wang <jasowang@...hat.com>
Subject: [PATCH] vhost: fix OOB in get_rx_bufs()

After batched used ring updating was introduced in commit e2b3b35eb989
("vhost_net: batch used ring update in rx"). We tend to batch heads in
vq->heads for more than one packet. But the quota passed to
get_rx_bufs() was not correctly limited, which can result a OOB write
in vq->heads.

        headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
                    vhost_len, &in, vq_log, &log,
                    likely(mergeable) ? UIO_MAXIOV : 1);

UIO_MAXIOV was still used which is wrong since we could have batched
used in vq->heads, this will cause OOB if the next buffer needs more
than 960 (1024 (UIO_MAXIOV) - 64 (VHOST_NET_BATCH)) heads after we've
batched 64 (VHOST_NET_BATCH) heads:

=============================================================================
BUG kmalloc-8k (Tainted: G    B            ): Redzone overwritten
-----------------------------------------------------------------------------

INFO: 0x00000000fd93b7a2-0x00000000f0713384. First byte 0xa9 instead of 0xcc
INFO: Allocated in alloc_pd+0x22/0x60 age=3933677 cpu=2 pid=2674
    kmem_cache_alloc_trace+0xbb/0x140
    alloc_pd+0x22/0x60
    gen8_ppgtt_create+0x11d/0x5f0
    i915_ppgtt_create+0x16/0x80
    i915_gem_create_context+0x248/0x390
    i915_gem_context_create_ioctl+0x4b/0xe0
    drm_ioctl_kernel+0xa5/0xf0
    drm_ioctl+0x2ed/0x3a0
    do_vfs_ioctl+0x9f/0x620
    ksys_ioctl+0x6b/0x80
    __x64_sys_ioctl+0x11/0x20
    do_syscall_64+0x43/0xf0
    entry_SYSCALL_64_after_hwframe+0x44/0xa9
INFO: Slab 0x00000000d13e87af objects=3 used=3 fp=0x          (null) flags=0x200000000010201
INFO: Object 0x0000000003278802 @offset=17064 fp=0x00000000e2e6652b

Fixing this by allocating UIO_MAXIOV + VHOST_NET_BATCH iovs for
vhost-net. This is done through set the limitation through
vhost_dev_init(), then set_owner can allocate the number of iov in a
per device manner.

Fixes: e2b3b35eb989 ("vhost_net: batch used ring update in rx")
Signed-off-by: Jason Wang <jasowang@...hat.com>
---
 drivers/vhost/net.c   | 3 ++-
 drivers/vhost/scsi.c  | 2 +-
 drivers/vhost/vhost.c | 7 ++++---
 drivers/vhost/vhost.h | 4 +++-
 drivers/vhost/vsock.c | 2 +-
 5 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 36f3d0f49e60..87af3b78d647 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -1336,7 +1336,8 @@ static int vhost_net_open(struct inode *inode, struct file *f)
 		n->vqs[i].rx_ring = NULL;
 		vhost_net_buf_init(&n->vqs[i].rxq);
 	}
-	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
+	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX,
+		       UIO_MAXIOV + VHOST_NET_BATCH);
 
 	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, EPOLLOUT, dev);
 	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, EPOLLIN, dev);
diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 8e10ab436d1f..a08ce2cda9f7 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -1623,7 +1623,7 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
 		vqs[i] = &vs->vqs[i].vq;
 		vs->vqs[i].vq.handle_kick = vhost_scsi_handle_kick;
 	}
-	vhost_dev_init(&vs->dev, vqs, VHOST_SCSI_MAX_VQ);
+	vhost_dev_init(&vs->dev, vqs, VHOST_SCSI_MAX_VQ, UIO_MAXIOV);
 
 	vhost_scsi_init_inflight(vs, NULL);
 
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 9f7942cbcbb2..1e739313d58e 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -390,9 +390,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
 		vq->indirect = kmalloc_array(UIO_MAXIOV,
 					     sizeof(*vq->indirect),
 					     GFP_KERNEL);
-		vq->log = kmalloc_array(UIO_MAXIOV, sizeof(*vq->log),
+		vq->log = kmalloc_array(dev->iov_limit, sizeof(*vq->log),
 					GFP_KERNEL);
-		vq->heads = kmalloc_array(UIO_MAXIOV, sizeof(*vq->heads),
+		vq->heads = kmalloc_array(dev->iov_limit, sizeof(*vq->heads),
 					  GFP_KERNEL);
 		if (!vq->indirect || !vq->log || !vq->heads)
 			goto err_nomem;
@@ -414,7 +414,7 @@ static void vhost_dev_free_iovecs(struct vhost_dev *dev)
 }
 
 void vhost_dev_init(struct vhost_dev *dev,
-		    struct vhost_virtqueue **vqs, int nvqs)
+		    struct vhost_virtqueue **vqs, int nvqs, int iov_limit)
 {
 	struct vhost_virtqueue *vq;
 	int i;
@@ -427,6 +427,7 @@ void vhost_dev_init(struct vhost_dev *dev,
 	dev->iotlb = NULL;
 	dev->mm = NULL;
 	dev->worker = NULL;
+	dev->iov_limit = iov_limit;
 	init_llist_head(&dev->work_list);
 	init_waitqueue_head(&dev->wait);
 	INIT_LIST_HEAD(&dev->read_list);
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 466ef7542291..2f76e3da9552 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -170,9 +170,11 @@ struct vhost_dev {
 	struct list_head read_list;
 	struct list_head pending_list;
 	wait_queue_head_t wait;
+	int iov_limit;
 };
 
-void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);
+void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs,
+		    int nvqs, int iov_limit);
 long vhost_dev_set_owner(struct vhost_dev *dev);
 bool vhost_dev_has_owner(struct vhost_dev *dev);
 long vhost_dev_check_owner(struct vhost_dev *);
diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index bc42d38ae031..fca2d6589ea0 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -531,7 +531,7 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file)
 	vsock->vqs[VSOCK_VQ_TX].handle_kick = vhost_vsock_handle_tx_kick;
 	vsock->vqs[VSOCK_VQ_RX].handle_kick = vhost_vsock_handle_rx_kick;
 
-	vhost_dev_init(&vsock->dev, vqs, ARRAY_SIZE(vsock->vqs));
+	vhost_dev_init(&vsock->dev, vqs, ARRAY_SIZE(vsock->vqs), UIO_MAXIOV);
 
 	file->private_data = vsock;
 	spin_lock_init(&vsock->send_pkt_list_lock);
-- 
2.17.1

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