Messages in this thread | | | Date | Fri, 28 Apr 2023 19:29:35 +0200 | Subject | Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default | From | David Hildenbrand <> |
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On 28.04.23 19:13, Lorenzo Stoakes wrote: > On Fri, Apr 28, 2023 at 07:05:38PM +0200, David Hildenbrand wrote: >> On 28.04.23 19:01, Lorenzo Stoakes wrote: >>> 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. >> >> Yes. But the issue here is rather dereferencing something that has already >> been freed, eventually leading to undefined behavior. >> > > Is that an issue with interrupts disabled though? Will block page tables being > removed and as Kirill says (sorry I maybe misinterpreted you) we should be ok.
Let's rule out page table freeing. If our VMA only spans a single page and falls into the same PMD as another VMA, an munmap() would not even free a single page table.
However, if unmapping a page (flushing the TLB) would imply an IPI as Kirill said, we'd be fine. I recall that that's not the case for all architectures, but I might be just wrong.
... and now I'll stop reading mails until Tuesday :)
-- Thanks,
David / dhildenb
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