[PATCH] vhost/scsi: Fix incorrect usage of get_user_pages_fast write parameter

From: Nicholas A. Bellinger
Date: Fri Oct 25 2013 - 14:25:20 EST

From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>

This patch addresses a long-standing bug where the get_user_pages_fast()
write parameter used for setting the underlying page table entry permission
bits was incorrectly set to write=1 for data_direction=DMA_TO_DEVICE, and
passed into get_user_pages_fast() via vhost_scsi_map_iov_to_sgl().

However, this parameter is intended to signal WRITEs to pinned userspace
PTEs for the virtio-scsi DMA_FROM_DEVICE -> READ payload case, and *not*
for the virtio-scsi DMA_TO_DEVICE -> WRITE payload case.

This bug would manifest itself as random process segmentation faults on
KVM host after repeated vhost starts + stops and/or with lots of vhost
endpoints + LUNs.

Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Cc: Asias He <asias@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.6+
Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
drivers/vhost/scsi.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index ce5221f..e663921 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -1056,7 +1056,7 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
if (data_direction != DMA_NONE) {
ret = vhost_scsi_map_iov_to_sgl(cmd,
&vq->iov[data_first], data_num,
- data_direction == DMA_TO_DEVICE);
+ data_direction == DMA_FROM_DEVICE);
if (unlikely(ret)) {
vq_err(vq, "Failed to map iov to sgl\n");
goto err_free;

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