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 Mon, 24 Aug 2020, Hugh Dickins wrote:
> On Sat, 22 Aug 2020, Hugh Dickins wrote:
> > On Sat, 22 Aug 2020, Sasha Levin wrote:
> > > 
> > > I've followed your instructions and backported the patches:
> > > 
> > > bbe98f9cadff ("khugepaged: khugepaged_test_exit() check
> > > mmget_still_valid()") - to all branches.
> > > f3f99d63a815 ("khugepaged: adjust VM_BUG_ON_MM() in
> > > __khugepaged_enter()") - to all branches.
> > > 59ea6d06cfa9 ("coredump: fix race condition between collapse_huge_page()
> > > and core dumping") - for 4.4.
> > 
> > That's saved me time, thanks a lot for doing that work, Sasha.
> > 
> > I've checked the results (haha, read on) and they're all fine,
> > but one minor flaw in bisectability: the added 4.4 backport of
> > "coredump: fix race condition..." adds a line (deleted by the next commit)
> > 	result = SCAN_ANY_PROCESS;
> > but neither "result" nor "SCAN_ANY_PROCESS" is defined in that tree,
> > so that intermediate step would generate an easily fixed build error.
> > 
> > FWIW - I don't know whether that's something to care about or not.
> 
> Ah, but I missed that this one that we originally held back from 5.8,
> did not in fact get re-added to 5.8: all the backport series have it,
> but today's 5.8.4-rc1 does not have it.
> 
> That's not a disaster - the series builds without it, and having its
> fix without the fixed commit is just odd, no more unsafe than before;
> but it should be re-added for a 5.8.4-rc2 or 5.8.5.

I see 5.8 is at 5.8.5-rc1 today, but the commit below still missing:
please re-add it, then we can all forget about it at last - 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;