Messages in this thread | | | Date | Wed, 27 Sep 2023 15:29:35 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI |
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>> +static int remap_anon_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, >> + struct vm_area_struct *dst_vma, >> + struct vm_area_struct *src_vma, >> + unsigned long dst_addr, unsigned long src_addr, >> + pte_t *dst_pte, pte_t *src_pte, >> + pte_t orig_dst_pte, pte_t orig_src_pte, >> + spinlock_t *dst_ptl, spinlock_t *src_ptl, >> + struct folio *src_folio) >> +{ >> + struct anon_vma *dst_anon_vma; >> + >> + double_pt_lock(dst_ptl, src_ptl); >> + >> + if (!pte_same(*src_pte, orig_src_pte) || >> + !pte_same(*dst_pte, orig_dst_pte) || >> + folio_test_large(src_folio) || >> + folio_estimated_sharers(src_folio) != 1) {
^ here you should check PageAnonExclusive. Please get rid of any implicit explicit/implcit mapcount checks.
>> + double_pt_unlock(dst_ptl, src_ptl); >> + return -EAGAIN; >> + } >> + >> + BUG_ON(!folio_test_anon(src_folio)); >> + >> + dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON; >> + WRITE_ONCE(src_folio->mapping, >> + (struct address_space *) dst_anon_vma);
I have some cleanups pending for page_move_anon_rmap(), that moves the SetPageAnonExclusive hunk out. Here we should be using page_move_anon_rmap() [or rather, folio_move_anon_rmap() after my cleanups]
I'll send them out soonish.
>> + WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, >> + dst_addr)); >> + >> + orig_src_pte = ptep_clear_flush(src_vma, src_addr, src_pte); >> + orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot); >> + orig_dst_pte = maybe_mkwrite(pte_mkdirty(orig_dst_pte), >> + dst_vma); > > I think there's still a theoretical issue here that you could fix by > checking for the AnonExclusive flag, similar to the huge page case. > > Consider the following scenario: > > 1. process P1 does a write fault in a private anonymous VMA, creating > and mapping a new anonymous page A1 > 2. process P1 forks and creates two children P2 and P3. afterwards, A1 > is mapped in P1, P2 and P3 as a COW page, with mapcount 3. > 3. process P1 removes its mapping of A1, dropping its mapcount to 2. > 4. process P2 uses vmsplice() to grab a reference to A1 with get_user_pages() > 5. process P2 removes its mapping of A1, dropping its mapcount to 1. > > If at this point P3 does a write fault on its mapping of A1, it will > still trigger copy-on-write thanks to the AnonExclusive mechanism; and > this is necessary to avoid P3 mapping A1 as writable and writing data > into it that will become visible to P2, if P2 and P3 are in different > security contexts. > > But if P3 instead moves its mapping of A1 to another address with > remap_anon_pte() which only does a page mapcount check, the > maybe_mkwrite() will directly make the mapping writable, circumventing > the AnonExclusive mechanism. >
Yes, can_change_pte_writable() contains the exact logic when we can turn something easily writable even if it wasn't writable before. which includes that PageAnonExclusive is set. (but with uffd-wp or softdirty tracking, there is more to consider)
-- Cheers,
David / dhildenb
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