null_blk: serialize configfs attribute stores with the lock

The NULLB_DEVICE_ATTR _store takes no lock: apply_fn attributes
(submit_queues, poll_queues) get dev->NAME written again after apply_fn
returns, outside its lock; APPLY=NULL attributes are entirely lockless.
configfs only serializes stores per-open-file, so concurrent stores on
separate fds race.

For apply_fn attributes, once one store's apply_fn has reconfigured the
hardware, a second (losing) store can still overwrite dev->NAME
afterwards. This leaves dev->submit_queues out of sync with the live
queue count, which is later caught by the WARN_ON_ONCE() in
null_map_queues().

For !apply_fn attributes, power_store()'s null_add_dev() validates and
builds the device under "lock" but only sets CONFIGURED afterwards. A store
slipping in during this window can change a field mid-setup -- for example,
zone_nr_conv can be pushed above nr_zones after it has already been
clamped, leading to an out-of-bounds dev->zones[] access.

Take "lock" in the macro around the apply_fn call, the CONFIGURED test and
the field write, and move it out of nullb_apply_submit_queues()/
nullb_apply_poll_queues() so both paths are covered once. This serializes
stores with power_store's setup and with each other.

Fixes: 45919fbfe1c4 ("null_blk: Enable modifying 'submit_queues' after an instance has been configured")
Suggested-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://patch.msgid.link/20260725022509.714271-10-wozizhi@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 14 deletions

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 962b0e05ce50..c7dfbec3e7d6 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -360,6 +360,7 @@ nullb_device_##NAME##_store(struct config_item *item, const char *page,	\
 	ret = nullb_device_##TYPE##_attr_store(&new_value, page, count);\
 	if (ret < 0)							\
 		return ret;						\
+	guard(mutex)(&lock);						\
 	if (apply_fn)							\
 		ret = apply_fn(dev, new_value);				\
 	else if (test_bit(NULLB_DEV_FL_CONFIGURED, &dev->flags)) 	\
@@ -430,25 +431,13 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 static int nullb_apply_submit_queues(struct nullb_device *dev,
 				     unsigned int submit_queues)
 {
-	int ret;
-
-	mutex_lock(&lock);
-	ret = nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
-	mutex_unlock(&lock);
-
-	return ret;
+	return nullb_update_nr_hw_queues(dev, submit_queues, dev->poll_queues);
 }
 
 static int nullb_apply_poll_queues(struct nullb_device *dev,
 				   unsigned int poll_queues)
 {
-	int ret;
-
-	mutex_lock(&lock);
-	ret = nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
-	mutex_unlock(&lock);
-
-	return ret;
+	return nullb_update_nr_hw_queues(dev, dev->submit_queues, poll_queues);
 }
 
 NULLB_DEVICE_ATTR(size, ulong, NULL);