[PATCH 6.18 1217/1518] null_blk: reject per-device queue resize for shared tag set

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

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

From: Zizhi Wo <redacted>

[ Upstream commit 1cdfe2fa62b48728a9b436fbbd3dbe4c11593e24 ]

When shared_tags is enabled, null_setup_tagset() makes the device use the
global tag_set, whose driver_data stays NULL. null_map_queues() therefore
falls back to the module-wide g_submit_queues/g_poll_queues instead of any
per-device value.

Resizing submit_queues or poll_queues via configfs on such a device calls
blk_mq_update_nr_hw_queues() on the shared set, shrinking
set->nr_hw_queues.  __blk_mq_realloc_hw_ctxs() only grows the
q->queue_hw_ctx[] allocation, so on shrink it merely exits and NULLs the
now-excess hctx slots. null_map_queues(), however, keeps mapping CPUs with
the unchanged g_submit_queues/g_poll_queues, so mq_map[] ends up pointing
at those NULLed hctx slots. blk_mq_map_swqueue() then dereferences the NULL
hctx (hctx->cpumask), crashing the kernel:

[  460.218374] KASAN: null-ptr-deref in range [0x0000000000000098-0x000000000000009f]
[  460.219003] CPU: 24 UID: 0 PID: 1492 Comm: sh Not tainted 7.2.0-rc2+ #67 PREEMPT(full)
[  460.219792] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014
[  460.220452] RIP: 0010:blk_mq_map_swqueue+0x4db/0x1430
......
[  460.228977] Call Trace:
[  460.229175]  <TASK>
[  460.229354]  blk_mq_update_nr_hw_queues+0xd49/0x11c0
[  460.229779]  ? __pfx_blk_mq_update_nr_hw_queues+0x10/0x10
[  460.230200]  nullb_update_nr_hw_queues+0x1a9/0x370 [null_blk]
[  460.230694]  nullb_device_submit_queues_store+0xd9/0x170 [null_blk]
[  460.231190]  ? __pfx_nullb_device_submit_queues_store+0x10/0x10 [null_blk]
[  460.231776]  ? configfs_write_iter+0x35c/0x4e0
[  460.232122]  configfs_write_iter+0x286/0x4e0
[  460.232460]  vfs_write+0x52d/0xd00
[  460.232779]  ? __x64_sys_openat+0x108/0x1d0
[  460.233106]  ? __pfx_vfs_write+0x10/0x10
[  460.233413]  ? fdget_pos+0x1cf/0x4c0
[  460.233745]  ? fput_close+0x133/0x190
[  460.234038]  ? __pfx_expand_files+0x10/0x10
[  460.234368]  ksys_write+0xfc/0x1d0

Reproducer:
modprobe null_blk shared_tags=1 submit_queues=64 poll_queues=1
mkdir /sys/kernel/config/nullb/dev
echo 1 > /sys/kernel/config/nullb/dev/power
echo 1 > /sys/kernel/config/nullb/dev/submit_queues

A per-device resize of a shared tag set is meaningless anyway, so reject it
with -EINVAL in nullb_update_nr_hw_queues() when the device is bound to the
global tag_set.

Fixes: 45919fbfe1c4 ("null_blk: Enable modifying 'submit_queues' after an instance has been configured")
Suggested-by: Nilay Shroff <redacted>
Assisted-by: Claude-Code:GLM-5.2
Signed-off-by: Zizhi Wo <redacted>
Reviewed-by: Nilay Shroff <redacted>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20260725022509.714271-8-wozizhi@huaweicloud.com (local)
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/block/null_blk/main.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index a65e52f6b48cb..50bc0b008b558 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c

@@ -382,6 +382,15 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
 	if (!dev->nullb)
 		return 0;
 
+	/*
+	 * A shared tag_set is mapped via the module-wide queue counts, so a
+	 * per-device resize is meaningless. On shrink it would also leave
+	 * mq_map[] pointing at NULLed hctx slots, causing a NULL deref in
+	 * blk_mq_map_swqueue(). Reject it.
+	 */
+	if (dev->shared_tags)
+		return -EINVAL;
+
 	/*
 	 * Make sure at least one submit queue exists.
 	 */

-- 
2.53.0