Re: Patch "khugepaged: khugepaged_test_exit() check mmget_still_valid()" has been added to the 5.8-stable tree

[Hugh Dickins <hughd@google.com>]

On Wed, 19 Aug 2020, gregkh@linuxfoundation.org wrote:
> 
> This is a note to let you know that I've just added the patch titled
> 
>     khugepaged: khugepaged_test_exit() check mmget_still_valid()
> 
> to the 5.8-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      khugepaged-khugepaged_test_exit-check-mmget_still_valid.patch
> and it can be found in the queue-5.8 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.

Please hold this one back for the moment: we shall want it, but syzbot
detected one place where it can lead to a VM_BUG_ON_MM().  The fix to
that is currently in Andrew's tree, but not yet in Linus's - when it
gets there, I'll send you its git commit id in reply to this mail.

This patch failed to apply to earlier releases: I'll send the fixup for
those at that time.  (Fixups for another patch to follow later today.)

Thanks,
Hugh

> 
> 
> From bbe98f9cadff58cdd6a4acaeba0efa8565dabe65 Mon Sep 17 00:00:00 2001
> From: Hugh Dickins <hughd@google.com>
> Date: Thu, 6 Aug 2020 23:26:25 -0700
> Subject: khugepaged: khugepaged_test_exit() check mmget_still_valid()
> 
> From: Hugh Dickins <hughd@google.com>
> 
> commit bbe98f9cadff58cdd6a4acaeba0efa8565dabe65 upstream.
> 
> Move collapse_huge_page()'s mmget_still_valid() check into
> khugepaged_test_exit() itself.  collapse_huge_page() is used for anon THP
> only, and earned its mmget_still_valid() check because it inserts a huge
> pmd entry in place of the page table's pmd entry; whereas
> collapse_file()'s retract_page_tables() or collapse_pte_mapped_thp()
> merely clears the page table's pmd entry.  But core dumping without mmap
> lock must have been as open to mistaking a racily cleared pmd entry for a
> page table at physical page 0, as exit_mmap() was.  And we certainly have
> no interest in mapping as a THP once dumping core.
> 
> Fixes: 59ea6d06cfa9 ("coredump: fix race condition between collapse_huge_page() and core dumping")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: <stable@vger.kernel.org>	[4.8+]
> Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008021217020.27773@eggly.anvils
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  mm/khugepaged.c |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(stru
>  
>  static inline int khugepaged_test_exit(struct mm_struct *mm)
>  {
> -	return atomic_read(&mm->mm_users) == 0;
> +	return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
>  }
>  
>  static bool hugepage_vma_check(struct vm_area_struct *vma,
> @@ -1100,9 +1100,6 @@ static void collapse_huge_page(struct mm
>  	 * handled by the anon_vma lock + PG_lock.
>  	 */
>  	mmap_write_lock(mm);
> -	result = SCAN_ANY_PROCESS;
> -	if (!mmget_still_valid(mm))
> -		goto out;
>  	result = hugepage_vma_revalidate(mm, address, &vma);
>  	if (result)
>  		goto out;
> 
> 
> Patches currently in stable-queue which might be from hughd@google.com are
> 
> queue-5.8/khugepaged-collapse_pte_mapped_thp-flush-the-right-range.patch
> queue-5.8/hugetlbfs-remove-call-to-huge_pte_alloc-without-i_mmap_rwsem.patch
> queue-5.8/khugepaged-khugepaged_test_exit-check-mmget_still_valid.patch
> queue-5.8/khugepaged-retract_page_tables-remember-to-test-exit.patch
> queue-5.8/khugepaged-collapse_pte_mapped_thp-protect-the-pmd-lock.patch
>