lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

While testing null_blk with configfs, echo 0 > poll_queues will trigger
following panic:

BUG: kernel NULL pointer dereference, address: 0000000000000010
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 27 UID: 0 PID: 920 Comm: bash Not tainted 6.15.0-02023-gadbdb95c8696-dirty #1238 PREEMPT(undef)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014
RIP: 0010:__bitmap_or+0x48/0x70
Call Trace:
 <TASK>
 __group_cpus_evenly+0x822/0x8c0
 group_cpus_evenly+0x2d9/0x490
 blk_mq_map_queues+0x1e/0x110
 null_map_queues+0xc9/0x170 [null_blk]
 blk_mq_update_queue_map+0xdb/0x160
 blk_mq_update_nr_hw_queues+0x22b/0x560
 nullb_update_nr_hw_queues+0x71/0xf0 [null_blk]
 nullb_device_poll_queues_store+0xa4/0x130 [null_blk]
 configfs_write_iter+0x109/0x1d0
 vfs_write+0x26e/0x6f0
 ksys_write+0x79/0x180
 __x64_sys_write+0x1d/0x30
 x64_sys_call+0x45c4/0x45f0
 do_syscall_64+0xa5/0x240
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Root cause is that numgrps is set to 0, and ZERO_SIZE_PTR is returned from
kcalloc(), and later ZERO_SIZE_PTR will be deferenced.

Fix the problem by checking numgrps first in group_cpus_evenly(), and
return NULL directly if numgrps is zero.

[[email protected]: also fix the non-SMP version]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 6a6dcae ("blk-mq: Build default queue map via group_cpus_evenly()")
Signed-off-by: Yu Kuai <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Cc: ErKun Yang <[email protected]>
Cc: John Garry <[email protected]>
Cc: Thomas Gleinxer <[email protected]>
Cc: "zhangyi (F)" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: YuKuai-huawei

lib/group_cpus.c

@@ -352,6 +352,9 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	int ret = -ENOMEM;
 	struct cpumask *masks = NULL;
 
+	if (numgrps == 0)
+		return NULL;
+
 	if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL))
 		return NULL;
 
@@ -426,8 +429,12 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
 #else /* CONFIG_SMP */
 struct cpumask *group_cpus_evenly(unsigned int numgrps)
 {
-	struct cpumask *masks = kcalloc(numgrps, sizeof(*masks), GFP_KERNEL);
+	struct cpumask *masks;
 
+	if (numgrps == 0)
+		return NULL;
+
+	masks = kcalloc(numgrps, sizeof(*masks), GFP_KERNEL);
 	if (!masks)
 		return NULL;