[v6,3/3,v4.9.y] coredump: fix race condition between collapse_huge_page() and core dumping

Commit Message

Ajay Kaher Aug. 4, 2019, 3:59 a.m. UTC

From: Andrea Arcangeli <[e-mail]>

coredump: fix race condition between collapse_huge_page() and core dumping

commit 59ea6d06cfa9247b586a695c21f94afa7183af74 upstream.

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 04f5866e41fb70 ("coredump: fix
race condition between mmget_not_zero()/get_task_mm() and core
dumping"), 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().

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: ba76149f47d8 ("thp: khugepaged")
Signed-off-by: Andrea Arcangeli <[e-mail]>
Reported-by: Michal Hocko <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
[Ajay: Just adjusted to apply on v4.9]
Signed-off-by: Ajay Kaher <[e-mail]>
---
 include/linux/mm.h | 4 ++++
 mm/khugepaged.c    | 3 +++
 2 files changed, 7 insertions(+)

Patch

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c239984..8852158 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1194,6 +1194,10 @@  void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
  * followed by taking the mmap_sem for writing before modifying the
  * vmas or anything the coredump pretends not to change from under it.
  *
+ * It also has to be called when mmgrab() is used in the context of
+ * the process, but then the mm_count refcount is transferred outside
+ * the context of the process to run down_write() on that pinned mm.
+ *
  * NOTE: find_extend_vma() called from GUP context is the only place
  * that can modify the "mm" (notably the vm_start/end) under mmap_sem
  * for reading and outside the context of the process, so it is also
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e0cfc3a..8217ee5 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1004,6 +1004,9 @@  static void collapse_huge_page(struct mm_struct *mm,
 	 * handled by the anon_vma lock + PG_lock.
 	 */
 	down_write(&mm->mmap_sem);
+	result = SCAN_ANY_PROCESS;
+	if (!mmget_still_valid(mm))
+		goto out;
 	result = hugepage_vma_revalidate(mm, address, &vma);
 	if (result)
 		goto out;