On Fri, Apr 28, 2023 at 06:51:46PM +0200, David Hildenbrand wrote: > On 28.04.23 18:39, Peter Xu wrote: > > On Fri, Apr 28, 2023 at 07:22:07PM +0300, Kirill A . Shutemov wrote: > > > On Fri, Apr 28, 2023 at 06:13:03PM +0200, David Hildenbrand wrote: > > > > On 28.04.23 18:09, Kirill A . Shutemov wrote: > > > > > On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote: > > > > > > On 28.04.23 17:34, David Hildenbrand wrote: > > > > > > > On 28.04.23 17:33, Lorenzo Stoakes wrote: > > > > > > > > On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote: > > > > > > > > > > > > > > > > > > > > > > Security is the primary case where we have historically closed uAPI > > > > > > > > > > > items. > > > > > > > > > > > > > > > > > > > > As this patch > > > > > > > > > > > > > > > > > > > > 1) Does not tackle GUP-fast > > > > > > > > > > 2) Does not take care of !FOLL_LONGTERM > > > > > > > > > > > > > > > > > > > > I am not convinced by the security argument in regard to this patch. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > If we want to sells this as a security thing, we have to block it > > > > > > > > > > *completely* and then CC stable. > > > > > > > > > > > > > > > > > > Regarding GUP-fast, to fix the issue there as well, I guess we could do > > > > > > > > > something similar as I did in gup_must_unshare(): > > > > > > > > > > > > > > > > > > If we're in GUP-fast (no VMA), and want to pin a !anon page writable, > > > > > > > > > fallback to ordinary GUP. IOW, if we don't know, better be safe. > > > > > > > > > > > > > > > > How do we determine it's non-anon in the first place? The check is on the > > > > > > > > VMA. We could do it by following page tables down to folio and checking > > > > > > > > folio->mapping for PAGE_MAPPING_ANON I suppose? > > > > > > > > > > > > > > PageAnon(page) can be called from GUP-fast after grabbing a reference. > > > > > > > See gup_must_unshare(). > > > > > > > > > > > > IIRC, PageHuge() can also be called from GUP-fast and could special-case > > > > > > hugetlb eventually, as it's table while we hold a (temporary) reference. > > > > > > Shmem might be not so easy ... > > > > > > > > > > page->mapping->a_ops should be enough to whitelist whatever fs you want. > > > > > > > > > > > > > The issue is how to stabilize that from GUP-fast, such that we can safely > > > > dereference the mapping. Any idea? > > > > > > > > At least for anon page I know that page->mapping only gets cleared when > > > > freeing the page, and we don't dereference the mapping but only check a > > > > single flag stored alongside the mapping. Therefore, PageAnon() is fine in > > > > GUP-fast context. > > > > > > What codepath you are worry about that clears ->mapping on pages with > > > non-zero refcount? > > > > > > I can only think of truncate (and punch hole). READ_ONCE(page->mapping) > > > and fail GUP_fast if it is NULL should be fine, no? > > > > > > I guess we should consider if the inode can be freed from under us and the > > > mapping pointer becomes dangling. But I think we should be fine here too: > > > VMA pins inode and VMA cannot go away from under GUP. > > > > Can vma still go away if during a fast-gup? > > > > So, after we grabbed the page and made sure the the PTE didn't change (IOW, > the PTE was stable while we processed it), the page can get unmapped (but > not freed, because we hold a reference) and the VMA can theoretically go > away (and as far as I understand, nothing stops the file from getting > deleted, truncated etc). > > So we might be looking at folio->mapping and the VMA is no longer there. > Maybe even the file is no longer there. > This shouldn't be an issue though right? Because after a pup call unlocks the mmap_lock we're in the same situation anyway. GUP doesn't generally guarantee the mapping remains valid, only pinning the underlying folio. I'm thinking of respinning with a gup_fast component then, if a_ops is sufficient to identify file systems. We'll just revert to slow path for non-FOLL_FAST_ONLY cases. This would at least cover both FOLL_LONGTERM angles and could provoke some further interesting discussion :) > -- > Thanks, > > David / dhildenb >
