1. 25 Apr, 2019 2 commits
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    • Linus Torvalds's avatar
      Linux 5.1-rc5 · dc4060a5
      Linus Torvalds authored
      dc4060a5
    • Linus Torvalds's avatar
      Merge branch 'page-refs' (page ref overflow) · 6b3a7077
      Linus Torvalds authored
      Merge page ref overflow branch.
      
      Jann Horn reported that he can overflow the page ref count with
      sufficient memory (and a filesystem that is intentionally extremely
      slow).
      
      Admittedly it's not exactly easy.  To have more than four billion
      references to a page requires a minimum of 32GB of kernel memory just
      for the pointers to the pages, much less any metadata to keep track of
      those pointers.  Jann needed a total of 140GB of memory and a specially
      crafted filesystem that leaves all reads pending (in order to not ever
      free the page references and just keep adding more).
      
      Still, we have a fairly straightforward way to limit the two obvious
      user-controllable sources of page references: direct-IO like page
      references gotten through get_user_pages(), and the splice pipe page
      duplication.  So let's just do that.
      
      * branch page-refs:
        fs: prevent page refcount overflow in pipe_buf_get
        mm: prevent get_user_pages() from overflowing page refcount
        mm: add 'try_get_page()' helper function
        mm: make page ref count overflow check tighter and more explicit
      6b3a7077