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. 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? I agree with you though, given the limited options we should convert the UAF into an unlikely deadlock. Jason
