tcm_loop: Add explict read buffer memset for SCF_SCSI_CONTROL_SG_IO_CDB

This patch addresses an issue with buggy userspace code sending I/O
via scsi-generic that does not explictly clear their associated read
buffers.  It adds an explict memset of the first SGL entry within
tcm_loop_new_cmd_map() for SCF_SCSI_CONTROL_SG_IO_CDB payloads that
are currently guaranteed to be a single SGL by target-core code.

This issue is a side effect of the v3.1-rc1 merge to remove the
extra memcpy between certain control CDB types using a contigious
+ cleared buffer in target-core, and performing a memcpy into the
SGL list within tcm_loop.

It was originally mainfesting itself by udev + scsi_id + scsi-generic
not properly setting up the expected /dev/disk/by-id/ symlinks because
the INQUIRY payload was containing extra bogus data preventing the
proper NAA IEEE WWN from being parsed by userspace.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/drivers/target/loopback/tcm_loop.c b/drivers/target/loopback/tcm_loop.c
index b15d8cbf630b..3c9c318f66ed 100644
--- a/drivers/target/loopback/tcm_loop.c
+++ b/drivers/target/loopback/tcm_loop.c
@@ -174,6 +174,24 @@ static int tcm_loop_new_cmd_map(struct se_cmd *se_cmd)
                 sgl_bidi = sdb->table.sgl;
                 sgl_bidi_count = sdb->table.nents;
         }
+        /*
+         * Because some userspace code via scsi-generic do not memset their
+         * associated read buffers, go ahead and do that here for type
+         * SCF_SCSI_CONTROL_SG_IO_CDB.  Also note that this is currently
+         * guaranteed to be a single SGL for SCF_SCSI_CONTROL_SG_IO_CDB
+         * by target core in transport_generic_allocate_tasks() ->
+         * transport_generic_cmd_sequencer().
+         */
+        if (se_cmd->se_cmd_flags & SCF_SCSI_CONTROL_SG_IO_CDB &&
+            se_cmd->data_direction == DMA_FROM_DEVICE) {
+                struct scatterlist *sg = scsi_sglist(sc);
+                unsigned char *buf = kmap(sg_page(sg)) + sg->offset;
+
+                if (buf != NULL) {
+                        memset(buf, 0, sg->length);
+                        kunmap(sg_page(sg));
+                }
+        }
 
         /* Tell the core about our preallocated memory */
         ret = transport_generic_map_mem_to_cmd(se_cmd, scsi_sglist(sc),