tcmu: prevent corruption when invalid data page requested

We will always have a page mapped for cmd data if it is
valid command. If the mapping does not exist then something
bad happened in userspace and it should not proceed. This
has us return VM_FAULT_SIGBUS when this happens instead of
returning a freshly allocated paged. The latter can cause
corruption because userspace might write the pages data
overwriting valid data or return it to the initiator.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_user.c

@@ -1342,7 +1342,6 @@ static int tcmu_find_mem_index(struct vm_area_struct *vma)
 static struct page *tcmu_try_get_block_page(struct tcmu_dev *udev, uint32_t dbi)
 {
 	struct page *page;
-	int ret;
 
 	mutex_lock(&udev->cmdr_lock);
 	page = tcmu_get_block_page(udev, dbi);
@@ -1352,42 +1351,12 @@ static struct page *tcmu_try_get_block_page(struct tcmu_dev *udev, uint32_t dbi)
 	}
 
 	/*
-	 * Normally it shouldn't be here:
-	 * Only when the userspace has touched the blocks which
-	 * are out of the tcmu_cmd's data iov[], and will return
-	 * one zeroed page.
+	 * Userspace messed up and passed in a address not in the
+	 * data iov passed to it.
 	 */
-	pr_warn("Block(%u) out of cmd's iov[] has been touched!\n", dbi);
-	pr_warn("Mostly it will be a bug of userspace, please have a check!\n");
-
-	if (dbi >= udev->dbi_thresh) {
-		/* Extern the udev->dbi_thresh to dbi + 1 */
-		udev->dbi_thresh = dbi + 1;
-		udev->dbi_max = dbi;
-	}
-
-	page = radix_tree_lookup(&udev->data_blocks, dbi);
-	if (!page) {
-		page = alloc_page(GFP_KERNEL | __GFP_ZERO);
-		if (!page) {
-			mutex_unlock(&udev->cmdr_lock);
-			return NULL;
-		}
-
-		ret = radix_tree_insert(&udev->data_blocks, dbi, page);
-		if (ret) {
-			mutex_unlock(&udev->cmdr_lock);
-			__free_page(page);
-			return NULL;
-		}
-
-		/*
-		 * Since this case is rare in page fault routine, here we
-		 * will allow the global_db_count >= tcmu_global_max_blocks
-		 * to reduce possible page fault call trace.
-		 */
-		atomic_inc(&global_db_count);
-	}
+	pr_err("Invalid addr to data block mapping  (dbi %u) on device %s\n",
+	       dbi, udev->name);
+	page = NULL;
 	mutex_unlock(&udev->cmdr_lock);
 
 	return page;
@@ -1422,7 +1391,7 @@ static int tcmu_vma_fault(struct vm_fault *vmf)
 		dbi = (offset - udev->data_off) / DATA_BLOCK_SIZE;
 		page = tcmu_try_get_block_page(udev, dbi);
 		if (!page)
-			return VM_FAULT_NOPAGE;
+			return VM_FAULT_SIGBUS;
 	}
 
 	get_page(page);