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

JIRA: https://issues.redhat.com/browse/RHEL-100484

commit df831e9
Author: Yu Kuai <[email protected]>
Date:   Thu Jun 19 21:26:55 2025 +0800

    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]>

Signed-off-by: Ming Lei <[email protected]>

Author: Ming Lei

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;