UAS: fix alignment of scatter/gather segments

This is the UAS version of

747668dbc061b3e62bc1982767a3a1f9815fcf0e
usb-storage: Set virt_boundary_mask to avoid SG overflows

We are not as likely to be vulnerable as storage, as it is unlikelier
that UAS is run over a controller without native support for SG,
but the issue exists.
The issue has been existing since the inception of the driver.

Fixes: 115bb1ffa54c ("USB: Add UAS driver")
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
index a6d68191c8617be7ecaeb00145947b425b698624..047c5922618f81d014e5f62c67249763492f609c 100644 (file)
--- a/drivers/usb/storage/uas.c
+++ b/drivers/usb/storage/uas.c
@@ -789,24 +789,33 @@ static int uas_slave_alloc(struct scsi_device *sdev)
 {
        struct uas_dev_info *devinfo =
                (struct uas_dev_info *)sdev->host->hostdata;
+       int maxp;
 
        sdev->hostdata = devinfo;
 
        /*
-        * USB has unusual DMA-alignment requirements: Although the
-        * starting address of each scatter-gather element doesn't matter,
-        * the length of each element except the last must be divisible
-        * by the Bulk maxpacket value.  There's currently no way to
-        * express this by block-layer constraints, so we'll cop out
-        * and simply require addresses to be aligned at 512-byte
-        * boundaries.  This is okay since most block I/O involves
-        * hardware sectors that are multiples of 512 bytes in length,
-        * and since host controllers up through USB 2.0 have maxpacket
-        * values no larger than 512.
+        * We have two requirements here. We must satisfy the requirements
+        * of the physical HC and the demands of the protocol, as we
+        * definitely want no additional memory allocation in this path
+        * ruling out using bounce buffers.
         *
-        * But it doesn't suffice for Wireless USB, where Bulk maxpacket
-        * values can be as large as 2048.  To make that work properly
-        * will require changes to the block layer.
+        * For a transmission on USB to continue we must never send
+        * a package that is smaller than maxpacket. Hence the length of each
+         * scatterlist element except the last must be divisible by the
+         * Bulk maxpacket value.
+        * If the HC does not ensure that through SG,
+        * the upper layer must do that. We must assume nothing
+        * about the capabilities off the HC, so we use the most
+        * pessimistic requirement.
+        */
+
+       maxp = usb_maxpacket(devinfo->udev, devinfo->data_in_pipe, 0);
+       blk_queue_virt_boundary(sdev->request_queue, maxp - 1);
+
+       /*
+        * The protocol has no requirements on alignment in the strict sense.
+        * Controllers may or may not have alignment restrictions.
+        * As this is not exported, we use an extremely conservative guess.
         */
        blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));