[PATCH v2 06/11] mtd: ubi: block: only claim UBI block devices in the partition scan

part_test_ubi() reads the first block and returns success for any
readable device, so a bare block device with no partition table (for
example a sandbox "host bind" image) is mis-detected as UBI once
CONFIG_UBI_BLOCK is enabled. part_get_info_ubi() then calls
ubi_get_volume_by_index(), which dereferences ubi_devices[0] without a
NULL check and crashes when no UBI device is attached.

Fail part_test_ubi() when no UBI device is attached, and guard the volume
lookup and print helpers against a NULL ubi_device, so the driver only
claims devices while UBI is present and never dereferences a missing one.

Fixes: aa5b67ce226 ("disk: support UBI partitions")
Signed-off-by: Daniel Golle <[email protected]>
---
 drivers/mtd/ubi/part.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/mtd/ubi/part.c b/drivers/mtd/ubi/part.c
index 6c017eb7299..e1bdd9e3008 100644
--- a/drivers/mtd/ubi/part.c
+++ b/drivers/mtd/ubi/part.c
@@ -19,6 +19,9 @@ static struct ubi_volume *ubi_get_volume_by_index(int vol_id)
 	struct ubi_device *ubi = get_ubi_device();
 	int i;
 
+	if (!ubi)
+		return NULL;
+
 	for (i = 0; i < (ubi->vtbl_slots + 1); i++) {
 		struct ubi_volume *volume = ubi->volumes[i];
 
@@ -66,6 +69,9 @@ static void __maybe_unused part_print_ubi(struct blk_desc *dev_desc)
 	struct ubi_device *ubi = get_ubi_device();
 	int i;
 
+	if (!ubi)
+		return;
+
 	for (i = 0; i < (ubi->vtbl_slots + 1); i++) {
 		struct ubi_volume *volume = ubi->volumes[i];
 
@@ -83,6 +89,9 @@ static int part_test_ubi(struct blk_desc *dev_desc)
 {
 	ALLOC_CACHE_ALIGN_BUFFER(unsigned char, buffer, dev_desc->blksz);
 
+	if (!get_ubi_device())
+		return -1;
+
 	if (blk_dread(dev_desc, 0, 1, (ulong *)buffer) != 1)
 		return -1;
 
-- 
2.55.0