Re: [PATCH] kernfs: Fix UAF in PSI polling when open file is released

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/8/20 9:46, Tejun Heo wrote:
> Hello,
> 
> On Mon, Aug 18, 2025 at 04:00:08PM +0800, Chen Ridong wrote:
>>> A potential solution is to make the lifecycles of cgroup_file_ctx and
>>> psi_trigger match the struct kernfs_open_file they're associated with.
>>> Maybe we could just get rid of the kernfs_release_file call in
>>> kernfs_drain_open_files?
>>>
>>
>> Hi, Tj, what do you think about this solution?
> 
> So, I think it's really fragile for a killed (drained) kernfs_open_file to
> be reused after the corresponding @kn is resurrected. Once killed, that file
> should stay dead. I think it'd be best if we can do this in a generic manner
> rather than trying to fix it only for poll.
> 
> kernfs_get_active() is the thing which gates active operations on the file.
> Maybe we can add a wrapper, say, kernfs_get_active_of(struct
> kernfs_open_file *of) which returns NULL if @of has already been killed or
> the underlying @kn can't be activated?
> 
> Thanks.
> 

Thank you Tj,

This is reasonable, I will try.

-- 
Best regards,
Ridong