[PATCH] misc: bcm_vk: Fix possible null-pointer dereferences in bcm_vk_read()

[Tuo Li]

In the function bcm_vk_read(), the pointer entry is checked, indicating 
that it can be NULL. If entry is NULL and rc is set to -EMSGSIZE, the 
following code may cause null-pointer dereferences:

  struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
  set_msg_id(&tmp_msg, entry->usr_msg_id);
  tmp_msg.size = entry->to_h_blks - 1;

To prevent these possible null-pointer dereferences, copy to_h_msg, 
usr_msg_id, and to_h_blks from iter into temporary variables, and return
these temporary variables to the application instead of accessing them
through a potentially NULL entry.

Signed-off-by: Tuo Li <islituo@xxxxxxxxx>
---
 drivers/misc/bcm-vk/bcm_vk_msg.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/bcm-vk/bcm_vk_msg.c b/drivers/misc/bcm-vk/bcm_vk_msg.c
index 1f42d1d5a630..665a3888708a 100644
--- a/drivers/misc/bcm-vk/bcm_vk_msg.c
+++ b/drivers/misc/bcm-vk/bcm_vk_msg.c
@@ -1010,6 +1010,9 @@ ssize_t bcm_vk_read(struct file *p_file,
 	struct device *dev = &vk->pdev->dev;
 	struct bcm_vk_msg_chan *chan = &vk->to_h_msg_chan;
 	struct bcm_vk_wkent *entry = NULL, *iter;
+	struct vk_msg_blk tmp_msg;
+	u32 tmp_usr_msg_id;
+	u32 tmp_blks;
 	u32 q_num;
 	u32 rsp_length;
 
@@ -1034,6 +1037,9 @@ ssize_t bcm_vk_read(struct file *p_file,
 					entry = iter;
 				} else {
 					/* buffer not big enough */
+					tmp_msg = iter->to_h_msg[0];
+					tmp_usr_msg_id = iter->usr_msg_id;
+					tmp_blks = iter->to_h_blks;
 					rc = -EMSGSIZE;
 				}
 				goto read_loop_exit;
@@ -1052,14 +1058,12 @@ ssize_t bcm_vk_read(struct file *p_file,
 
 		bcm_vk_free_wkent(dev, entry);
 	} else if (rc == -EMSGSIZE) {
-		struct vk_msg_blk tmp_msg = entry->to_h_msg[0];
-
 		/*
 		 * in this case, return just the first block, so
 		 * that app knows what size it is looking for.
 		 */
-		set_msg_id(&tmp_msg, entry->usr_msg_id);
-		tmp_msg.size = entry->to_h_blks - 1;
+		set_msg_id(&tmp_msg, tmp_usr_msg_id);
+		tmp_msg.size = tmp_blks - 1;
 		if (copy_to_user(buf, &tmp_msg, VK_MSGQ_BLK_SIZE) != 0) {
 			dev_err(dev, "Error return 1st block in -EMSGSIZE\n");
 			rc = -EFAULT;
-- 
2.43.0