Re: [PATCH V4 7/9] null_blk: reject per-device queue resize for shared tag set

[Nilay Shroff <nilay@linux.ibm.com>]

On 7/9/26 3:34 PM, Zizhi Wo wrote:
> From: Zizhi Wo <wozizhi@huawei.com>
> 
> 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")
> Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
> ---
>   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 6d30591abb28..340ecc0a331e 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/drivers/block/null_blk/main.c
> @@ -380,10 +380,19 @@ static int nullb_update_nr_hw_queues(struct nullb_device *dev,
>   	int ret, nr_hw_queues;
>   
>   	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->nullb->tag_set == &tag_set)
> +		return -EINVAL;
> +

Wouldn't it be simpler to check dev->shared_tags here instead? Since the restriction
is specifically for devices configured with shared tags, that seems a bit easier to
reason about than checking whether dev->nullb->tag_set points to the global tag_set.
Something like this:

if (dev->shared_tags)
         return -EINVAL;
Thanks,
--Nilay