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

[Tejun Heo <tj@kernel.org>]

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.

Thanks.

-- 
tejun