Re: kernfs: possible deadlock between of->mutex and mmap_sem

[Sasha Levin <sasha.levin@oracle.com>]

On 03/03/2014 05:39 PM, Tejun Heo wrote:
> On Fri, Feb 28, 2014 at 08:14:45PM -0500, Sasha Levin wrote:
>> Hi all,
>>
>> I've stumbled on the following while fuzzing with trinity inside a
>> KVM tools running the latest -next kernel.
>>
>> We deal with files that have an mmap op by giving them a different
>> locking class than the files which don't due to mmap_sem nesting
>> being different for those files.
>>
>> We assume that for mmap supporting files, of->mutex will be nested
>> inside mm->mmap_sem. However, this is not always the case. Consider
>> the following:
>>
>> 	kernfs_fop_write()
>> 		copy_from_user()
>> 			might_fault()
>>
>> might_fault() suggests that we may lock mm->mmap_sem, which causes a
>> reverse lock nesting of mm->mmap_sem inside of of->mutex.
>>
>> I'll send a patch to fix it some time next week unless someone beats me to it :)
>
> How are you planning to fix it?  Prolly the right thing to do would be
> caching atomic_write_len in open_file and copy data before grabbing
> any locks.

I've actually didn't have a plan when I wrote that, was just planning on putting effort into it.


Thanks,
Sasha