null_blk: reject per-device queue resize for shared tag set

In the Linux kernel, the following vulnerability has been resolved: null_blk: reject per-device queue resize for shared tag set 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.