[PATCH 3.6 2/2] target: use a bounce buffer in transport_kmap_data_sg for 0 or 1-page sglist

[Paolo Bonzini]

This patch started with the aim of fixing START STOP UNIT to a PSCSI
device.  Right now, commands with a zero-size payload are skipped
completely.  This is wrong; such commands should be passed down to the
device and processed normally.  As a hint of this, we have a hack to
clear a unit attention state on a zero-size REQUEST SENSE.

The problem with fixing this, is that we do not have a page to pass
back to the caller of transport_kmap_data_sg.  But this is just
a special case of a more general overflow that could happen after
using transport_kmap_data_sg.  For example, the REQUEST_SENSE handler
expects 8 bytes, but if you send a CDB with a small allocation length
(e.g. 4 bytes).  you might end up with a single-page sglist and a large
sg->offset.  This would make buf[7] already inaccessible.

Luckily, transport_kmap_data_sg is not called on the I/O path, so we can
simply allocate a one-page bounce buffer there, which indeed also takes
care of zero-sized transfers.
---
 drivers/target/target_core_transport.c |   62 ++++++++++++++-----------------
 1 files changed, 28 insertions(+), 34 deletions(-)

diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
index 09028af..a77c8aa 100644
--- a/drivers/target/target_core_transport.c
+++ b/drivers/target/target_core_transport.c
@@ -2181,20 +2181,32 @@ EXPORT_SYMBOL(transport_generic_map_mem_to_cmd);
 
 void *transport_kmap_data_sg(struct se_cmd *cmd)
 {
+	u32 npages = DIV_ROUND_UP(cmd->data_length, PAGE_SIZE);
 	struct scatterlist *sg = cmd->t_data_sg;
 	struct page **pages;
 	int i;
 
-	BUG_ON(!sg);
+	BUG_ON(!sg && npages > 0);
+
 	/*
 	 * We need to take into account a possible offset here for fabrics like
 	 * tcm_loop who may be using a contig buffer from the SCSI midlayer for
 	 * control CDBs passed as SGLs via transport_generic_map_mem_to_cmd()
+	 *
+	 * This could cause overflows if the buffer is too small for the caller.
+	 * For example, the REQUEST_SENSE handler expects 8 bytes, but it is
+	 * possible to send a CDB with a small allocation length (e.g. 4 bytes).
+	 * In this case, we could have a single-page sglist with a large offset,
+	 * so that buf[7] is already inaccessible.
+	 *
+	 * But transport_kmap_data_sg is not called on the I/O path, so we can
+	 * simply allocate a one-page bounce buffer here.  This also takes care
+	 * of the case of zero-sized transfers.
 	 */
-	if (!cmd->t_data_nents)
-		return NULL;
-	else if (cmd->t_data_nents == 1)
-		return kmap(sg_page(sg)) + sg->offset;
+	if (npages <= 1) {
+		cmd->t_data_vmap = kzalloc(PAGE_SIZE, GFP_KERNEL);
+		return cmd->t_data_vmap;
+	}
 
 	/* >1 page. use vmap */
 	pages = kmalloc(sizeof(*pages) * cmd->t_data_nents, GFP_KERNEL);
@@ -2217,14 +2229,18 @@ EXPORT_SYMBOL(transport_kmap_data_sg);
 
 void transport_kunmap_data_sg(struct se_cmd *cmd)
 {
-	if (!cmd->t_data_nents) {
-		return;
-	} else if (cmd->t_data_nents == 1) {
-		kunmap(sg_page(cmd->t_data_sg));
-		return;
-	}
+	u32 npages = DIV_ROUND_UP(cmd->data_length, PAGE_SIZE);
+	if (npages <= 1) {
+		if (npages) {
+			struct scatterlist *sg = cmd->t_data_sg;
+			u8 *dest = kmap(sg_page(sg));
+			memcpy(dest + sg->offset, cmd->t_data_vmap, sg->length);
+			kunmap(sg_page(sg));
+		}
+		kfree(cmd->t_data_vmap);
+	} else
+		vunmap(cmd->t_data_vmap);
 
-	vunmap(cmd->t_data_vmap);
 	cmd->t_data_vmap = NULL;
 }
 EXPORT_SYMBOL(transport_kunmap_data_sg);
@@ -2290,28 +2306,6 @@ int transport_generic_new_cmd(struct se_cmd *cmd)
 		if (ret < 0)
 			goto out_fail;
 	}
-	/*
-	 * If this command doesn't have any payload and we don't have to call
-	 * into the fabric for data transfers, go ahead and complete it right
-	 * away.
-	 */
-	if (!cmd->data_length) {
-		spin_lock_irq(&cmd->t_state_lock);
-		cmd->t_state = TRANSPORT_COMPLETE;
-		cmd->transport_state |= CMD_T_ACTIVE;
-		spin_unlock_irq(&cmd->t_state_lock);
-
-		if (cmd->t_task_cdb[0] == REQUEST_SENSE) {
-			u8 ua_asc = 0, ua_ascq = 0;
-
-			core_scsi3_ua_clear_for_request_sense(cmd,
-					&ua_asc, &ua_ascq);
-		}
-
-		INIT_WORK(&cmd->work, target_complete_ok_work);
-		queue_work(target_completion_wq, &cmd->work);
-		return 0;
-	}
 
 	atomic_inc(&cmd->t_fe_count);
 
-- 
1.7.1

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