Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path

Jan Kara
2022-10-03T08:22:13Z

On Fri 30-09-22 15:06:47, Jason Gunthorpe wrote:
> On Fri, Sep 30, 2022 at 10:56:27AM -0700, Dan Williams wrote:
> > Jan Kara wrote:
> > [..]
> > > I agree this is doable but there's the nasty sideeffect that inode reclaim
> > > may block for abitrary time waiting for page pinning. If the application
> > > that has pinned the page requires __GFP_FS memory allocation to get to a
> > > point where it releases the page, we even have a deadlock possibility.
> > > So it's better than the UAF issue but still not ideal.
> >
> > I expect VMA pinning would have similar deadlock exposure if pinning a
> > VMA keeps the inode allocated. Anything that puts a page-pin release
> > dependency in the inode freeing path can potentially deadlock a reclaim
> > event that depends on that inode being freed.
>
> I think the desire would be to go from the VMA to an inode_get and
> hold the inode reference for the from the pin_user_pages() to the
> unpin_user_page(), ie prevent it from being freed in the first place.

Yes, that was the idea how to avoid UAF problems.

> It is a fine idea, the trouble is just the high complexity to get
> there.
>
> However, I wonder if the trucate/hole punch paths have the same
> deadlock problem?

Do you mean someone requiring say truncate(2) to complete on file F in
order to unpin pages of F? That is certainly a deadlock but it has always
worked this way for DAX so at least applications knowingly targetted at DAX
will quickly notice and avoid such unwise dependency ;).

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR