usb: storage: Fix memory leak in USB bulk transport

A kernel memory leak was identified by the 'ioctl_sg01' test from Linux
Test Project (LTP). The following bytes were mainly observed: 0x53425355.

When USB storage devices incorrectly skip the data phase with status data,
the code extracts/validates the CSW from the sg buffer, but fails to clear
it afterwards. This leaves status protocol data in srb's transfer buffer,
such as the US_BULK_CS_SIGN 'USBS' signature observed here. Thus, this can
lead to USB protocols leaks to user space through SCSI generic (/dev/sg*)
interfaces, such as the one seen here when the LTP test requested 512 KiB.

Fix the leak by zeroing the CSW data in srb's transfer buffer immediately
after the validation of devices that skip data phase.

Note: Differently from CVE-2018-1000204, which fixed a big leak by zero-
ing pages at allocation time, this leak occurs after allocation, when USB
protocol data is written to already-allocated sg pages.

Fixes: a45b599ad808 ("scsi: sg: allocate with __GFP_ZERO in sg_build_indirect()")
Cc: stable <[e-mail]>
Signed-off-by: Desnes Nunes <[e-mail]>
Reviewed-by: Alan Stern <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/storage/transport.c

···
 						US_BULK_CS_WRAP_LEN &&
 					bcs->Signature ==
 						cpu_to_le32(US_BULK_CS_SIGN)) {
+				unsigned char buf[US_BULK_CS_WRAP_LEN];
+
 				usb_stor_dbg(us, "Device skipped data phase\n");
+
+				/*
+				 * Devices skipping data phase might leave CSW data in srb's
+				 * transfer buffer. Zero it to prevent USB protocol leakage.
+				 */
+				sg = NULL;
+				offset = 0;
+				memset(buf, 0, sizeof(buf));
+				if (usb_stor_access_xfer_buf(buf,
+						US_BULK_CS_WRAP_LEN, srb, &sg,
+						&offset, TO_XFER_BUF) !=
+							US_BULK_CS_WRAP_LEN)
+					usb_stor_dbg(us, "Failed to clear CSW data\n");
+
 				scsi_set_resid(srb, transfer_length);
 				goto skipped_data_phase;
 			}