md: raid10: Fix null pointer dereference in fix_read_error()

Such NULL pointer dereference can occur when the driver was fixing the
read errors/bad blocks and the disk was physically removed
causing a system crash. This patch check if the
rcu_dereference() returns valid rdev before accessing it in fix_read_error().

Cc: stable@kernel.org
Signed-off-by: Prasanna S. Panchamukhi <prasanna.panchamukhi@riverbed.com>
Signed-off-by: Rob Becker <rbecker@riverbed.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -1482,14 +1482,14 @@ static void fix_read_error(conf_t *conf, mddev_t *mddev, r10bio_t *r10_bio)
 	int sectors = r10_bio->sectors;
 	mdk_rdev_t*rdev;
 	int max_read_errors = atomic_read(&mddev->max_corr_read_errors);
+	int d = r10_bio->devs[r10_bio->read_slot].devnum;
 
 	rcu_read_lock();
-	{
-		int d = r10_bio->devs[r10_bio->read_slot].devnum;
+	rdev = rcu_dereference(conf->mirrors[d].rdev);
+	if (rdev) { /* If rdev is not NULL */
 		char b[BDEVNAME_SIZE];
 		int cur_read_error_count = 0;
 
-		rdev = rcu_dereference(conf->mirrors[d].rdev);
 		bdevname(rdev->bdev, b);
 
 		if (test_bit(Faulty, &rdev->flags)) {
@@ -1530,7 +1530,7 @@ static void fix_read_error(conf_t *conf, mddev_t *mddev, r10bio_t *r10_bio)
 
 		rcu_read_lock();
 		do {
-			int d = r10_bio->devs[sl].devnum;
+			d = r10_bio->devs[sl].devnum;
 			rdev = rcu_dereference(conf->mirrors[d].rdev);
 			if (rdev &&
 			    test_bit(In_sync, &rdev->flags)) {
@@ -1564,7 +1564,7 @@ static void fix_read_error(conf_t *conf, mddev_t *mddev, r10bio_t *r10_bio)
 		rcu_read_lock();
 		while (sl != r10_bio->read_slot) {
 			char b[BDEVNAME_SIZE];
-			int d;
+
 			if (sl==0)
 				sl = conf->copies;
 			sl--;
@@ -1601,7 +1601,7 @@ static void fix_read_error(conf_t *conf, mddev_t *mddev, r10bio_t *r10_bio)
 		}
 		sl = start;
 		while (sl != r10_bio->read_slot) {
-			int d;
+
 			if (sl==0)
 				sl = conf->copies;
 			sl--;