[PATCH v2] scsi: target: tcmu: Fix crash on ARM during cmd completion

[Bodo Stroesser <bstroesser@ts.fujitsu.com>]

If tcmu_handle_completions() has to process a padding shorter
than sizeof(struct tcmu_cmd_entry), the current call to
tcmu_flush_dcache_range() with sizeof(struct tcmu_cmd_entry) as
length param is wrong and causes crashes on e.g. ARM, because
tcmu_flush_dcache_range() in this case calls
	flush_dcache_page(vmalloc_to_page(start));
with start being an invalid address above the end of the
vmalloc'ed area.

The fix is to use the minimum of remaining ring space and
sizeof(struct tcmu_cmd_entry) as the length param.

The patch was tested on kernel 4.19.118.

See https://bugzilla.kernel.org/show_bug.cgi?id=208045#c10

Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Tested-by: JiangYu <lnsyyj@hotmail.com>
Acked-by: Mike Christie <michael.christie@oracle.com>
---

v2: Description fixed (maximum --> minimum) and
    added Acked-by.

 drivers/target/target_core_user.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 3885ca532f8f..82e476d48194 100644
--- a/drivers/target/target_core_user.c
+++ b/drivers/target/target_core_user.c
@@ -1221,7 +1221,14 @@ static unsigned int tcmu_handle_completions(struct tcmu_dev *udev)
 
 		struct tcmu_cmd_entry *entry = (void *) mb + CMDR_OFF + udev->cmdr_last_cleaned;
 
-		tcmu_flush_dcache_range(entry, sizeof(*entry));
+		/*
+		 * Flush max. up to end of cmd ring, since current entry might
+		 * be a padding that is shorter than sizeof(*entry)
+		 */
+		size_t ring_left = head_to_end(udev->cmdr_last_cleaned,
+					       udev->cmdr_size);
+		tcmu_flush_dcache_range(entry, ring_left < sizeof(*entry) ?
+					ring_left : sizeof(*entry));
 
 		if (tcmu_hdr_get_op(entry->hdr.len_op) == TCMU_OP_PAD) {
 			UPDATE_HEAD(udev->cmdr_last_cleaned,
-- 
2.12.3