Re: [PATCH] lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jun 19, 2025 at 04:29:41PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> 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(), then __group_cpus_evenly() will deference the
> ZERO_SIZE_PTR.
> 
> Fix the problem by checking kcalloc() return value with ZERO_OR_NULL_PTR,
> and NULL will be returned to caller.
> 
> Fixes: 6a6dcae8f486 ("blk-mq: Build default queue map via group_cpus_evenly()")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  lib/group_cpus.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index ee272c4cefcc..5e243946ee4e 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -363,7 +363,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
>  		goto fail_npresmsk;
>  
>  	masks = kcalloc(numgrps, sizeof(*masks), GFP_KERNEL);
> -	if (!masks)
> +	if (ZERO_OR_NULL_PTR(masks))
>  		goto fail_node_to_cpumask;

I'd suggest to check 'numgrps' explicitly, which is more readable than
ZERO_PTR.


Thanks,
Ming