Re: [PATCH v2] lib/group_cpus: fix NULL pointer dereference from group_cpus_evenly()

[John Garry <john.g.garry@oracle.com>]

On 19/06/2025 14:26, 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 numgrps first in group_cpus_evenly(), and
> return NULL directly if numgrps is zero.
> 
> Fixes: 6a6dcae8f486 ("blk-mq: Build default queue map via group_cpus_evenly()")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
> Changes in v2:
>   - check numgrps directly.
> 
>   lib/group_cpus.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index ee272c4cefcc..d68407e3601e 100644
> --- a/lib/group_cpus.c
> +++ b/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;
>   

There's a non-SMP version of group_cpus_evenly() at the bottom of 
lib/group_cpus.c

It also does a kcalloc(numgrps, ...)

Does that need to be fixed as well?