On Fri, Jun 07, 2019 at 04:15:58PM +0000, Andrea Arcangeli wrote:
> When fixing the race conditions between the coredump and the mmap_sem
> holders outside the context of the process, we focused on
> mmget_not_zero()/get_task_mm() callers in commit
> 04f5866e41fb70690e28397487d8bd8eea7d712a, but those aren't the only
> cases where the mmap_sem can be taken outside of the context of the
> process as Michal Hocko noticed while backporting that commit to
> older -stable kernels.
>
> If mmgrab() is called in the context of the process, but then the
> mm_count reference is transferred outside the context of the process,
> that can also be a problem if the mmap_sem has to be taken for writing
> through that mm_count reference.
>
> khugepaged registration calls mmgrab() in the context of the process,
> but the mmap_sem for writing is taken later in the context of the
> khugepaged kernel thread.
>
> collapse_huge_page() after taking the mmap_sem for writing doesn't
> modify any vma, so it's not obvious that it could cause a problem to
> the coredump, but it happens to modify the pmd in a way that breaks an
> invariant that pmd_trans_huge_lock() relies upon. collapse_huge_page()
> needs the mmap_sem for writing just to block concurrent page faults
> that call pmd_trans_huge_lock().
>
> Specifically the invariant that "!pmd_trans_huge()" cannot become
> a "pmd_trans_huge()" doesn't hold while collapse_huge_page() runs.
>
> The coredump will call __get_user_pages() without mmap_sem for
> reading, which eventually can invoke a lockless page fault which will
> need a functional pmd_trans_huge_lock().
>
> So collapse_huge_page() needs to use mmget_still_valid() to check it's
> not running concurrently with the coredump... as long as the coredump
> can invoke page faults without holding the mmap_sem for reading.
>
> This has "Fixes: khugepaged" to facilitate backporting, but in my view
> it's more a bug in the coredump code that will eventually have to be
> rewritten to stop invoking page faults without the mmap_sem for
> reading. So the long term plan is still to drop all
> mmget_still_valid().
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: ba76149f47d8 ("thp: khugepaged")
> Reported-by: Michal Hocko <mhocko@xxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
--
Kirill A. Shutemov
