[PATCH] CVE fix for xen-blkback allowing DISCARD operations on read-only disks.

[PATCH] CVE fix for xen-blkback allowing DISCARD operations on read-only disks.

[Konrad Rzeszutek Wilk]

Please put the following patch:
 [PATCH] xen/blkback: Check device permissions before allowing

in your tree for Linus. It fixes CVE-2013-2140.

The bug is that if a system admin provides a disk (which supports
the discard aka TRIM or SCSI UNMAP) to a guest as read-only - there
are no checks done. Which means that the OS can destroy the data.

The likehood of somebody using 'ro' disks I think is small - but
there is probably one person who does it and would be unhappy that
a guest OS can destroy the underlaying data.

[PATCH] xen/blkback: Check device permissions before allowing OP_DISCARD

[Konrad Rzeszutek Wilk]

We need to make sure that the device is not RO or that
the request is not past the number of sectors we want to
issue the DISCARD operation for.

This fixes CVE-2013-2140.

Cc: stable@vger.kernel.org
Acked-by: Jan Beulich <JBeulich@suse.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
[v1: Made it pr_warn instead of pr_debug]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 drivers/block/xen-blkback/blkback.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index eea2c69..11ca405 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -881,7 +881,18 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	int status = BLKIF_RSP_OKAY;
 	struct block_device *bdev = blkif->vbd.bdev;
 	unsigned long secure;
+	struct phys_req preq;
+
+	preq.sector_number = req->u.discard.sector_number;
+	preq.nr_sects      = req->u.discard.nr_sectors;
 
+	err = xen_vbd_translate(&preq, blkif, WRITE);
+	if (err) {
+		pr_warn(DRV_PFX "access denied: DISCARD [%llu->%llu] on dev=%04x\n",
+			preq.sector_number,
+			preq.sector_number + preq.nr_sects, blkif->vbd.pdevice);
+		goto fail_response;
+	}
 	blkif->st_ds_req++;
 
 	xen_blkif_get(blkif);
@@ -892,7 +903,7 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	err = blkdev_issue_discard(bdev, req->u.discard.sector_number,
 				   req->u.discard.nr_sectors,
 				   GFP_KERNEL, secure);
-
+fail_response:
 	if (err == -EOPNOTSUPP) {
 		pr_debug(DRV_PFX "discard op failed, not supported\n");
 		status = BLKIF_RSP_EOPNOTSUPP;
-- 
1.8.1.4