[PATCH 7.1 100/228] s390/vfio_ccw: Fix out of bounds check on CCW array

7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Farman <[email protected]>

commit a005b7f1a491ffda61bff0fd0f6548f8986fb977 upstream.

The routine ccwchain_calc_length() counts the number of channel
command words (CCWs) that are chained together in a single channel
program, and rejects anything larger than CCWCHAIN_LEN_MAX (256) CCWs.

The loop itself is "do..while (count < 257)", and while the logic in
is_cpa_within_range() correctly adjusts between the 0-index array of
CCWs and the count of CCWs starting at 1, this means it would look
at a possible 257th CCW before ending the loop and (correctly)
returning an error.

Fix this by restructuring the loop to break as soon as 256 CCWs
(thus indexes 0-255) are examined, without looking at memory
outside the range.

Fixes: 0a19e61e6d4c ("vfio: ccw: introduce channel program interfaces")
Cc: [email protected]
Reviewed-by: Matthew Rosato <[email protected]>
Signed-off-by: Eric Farman <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 drivers/s390/cio/vfio_ccw_cp.c |   17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

--- a/drivers/s390/cio/vfio_ccw_cp.c
+++ b/drivers/s390/cio/vfio_ccw_cp.c
@@ -377,11 +377,9 @@ static void ccwchain_cda_free(struct ccw
 static int ccwchain_calc_length(u64 iova, struct channel_program *cp)
 {
 	struct ccw1 *ccw = cp->guest_cp;
-	int cnt = 0;
-
-	do {
-		cnt++;
+	int cnt;
 
+	for (cnt = 1; cnt <= CCWCHAIN_LEN_MAX; cnt++, ccw++) {
 		/*
 		 * We want to keep counting if the current CCW has the
 		 * command-chaining flag enabled, or if it is a TIC CCW
@@ -391,15 +389,10 @@ static int ccwchain_calc_length(u64 iova
 		 * after the TIC, depending on the results of its operation.
 		 */
 		if (!ccw_is_chain(ccw) && !is_tic_within_range(ccw, iova, cnt))
-			break;
-
-		ccw++;
-	} while (cnt < CCWCHAIN_LEN_MAX + 1);
-
-	if (cnt == CCWCHAIN_LEN_MAX + 1)
-		cnt = -EINVAL;
+			return cnt;
+	}
 
-	return cnt;
+	return -EINVAL;
 }
 
 static int tic_target_chain_exists(struct ccw1 *tic, struct channel_program *cp)