s390/dasd: Fix read for ESE with blksize < 4k

When reading unformatted tracks on ESE devices, the corresponding memory
areas are simply set to zero for each segment. This is done incorrectly
for blocksizes < 4096.

There are two problems. First, the increment of dst is done using the
counter of the loop (off), which is increased by blksize every
iteration. This leads to a much bigger increment for dst as actually
intended. Second, the increment of dst is done before the memory area
is set to 0, skipping a significant amount of bytes of memory.

This leads to illegal overwriting of memory and ultimately to a kernel
panic.

This is not a problem with 4k blocksize because
blk_queue_max_segment_size is set to PAGE_SIZE, always resulting in a
single iteration for the inner segment loop (bv.bv_len == blksize). The
incorrectly used 'off' value to increment dst is 0 and the correct
memory area is used.

In order to fix this for blksize < 4k, increment dst correctly using the
blksize and only do it at the end of the loop.

Fixes: 5e2b17e ("s390/dasd: Add dynamic formatting support for ESE volumes")
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://lore.kernel.org/r/20220505141733.1989450-4-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: hoeppnerj

drivers/s390/block/dasd_eckd.c

@@ -3285,12 +3285,11 @@ static int dasd_eckd_ese_read(struct dasd_ccw_req *cqr, struct irb *irb)
 				cqr->proc_bytes = blk_count * blksize;
 				return 0;
 			}
-			if (dst && !skip_block) {
-				dst += off;
+			if (dst && !skip_block)
 				memset(dst, 0, blksize);
-			} else {
+			else
 				skip_block--;
-			}
+			dst += blksize;
 			blk_count++;
 		}
 	}