[PATCH 1/1] coredump: fix race condition between collapse_huge_page() and core dumping

[Andrea Arcangeli <aarcange@redhat.com>]

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@vger.kernel.org>
Fixes: ba76149f47d8 ("thp: khugepaged")
Reported-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 include/linux/sched/mm.h | 4 ++++
 mm/khugepaged.c          | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index a3fda9f024c3..4a7944078cc3 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -54,6 +54,10 @@ static inline void mmdrop(struct mm_struct *mm)
  * 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 a335f7c1fac4..0f7419938008 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;