mm/hmm: fix race between hmm_mirror_unregister() and mmu_notifier callback

In hmm_mirror_unregister(), mm->hmm is set to NULL and then
mmu_notifier_unregister_no_release() is called.  That creates a small
window where mmu_notifier can call mmu_notifier_ops with mm->hmm equal to
NULL.  Fix this by first unregistering mmu notifier callbacks and then
setting mm->hmm to NULL.

Similarly in hmm_register(), set mm->hmm before registering mmu_notifier
callbacks so callback functions always see mm->hmm set.

Link: http://lkml.kernel.org/r/20181019160442.18723-4-jglisse@redhat.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 15 deletions

diff --git a/mm/hmm.c b/mm/hmm.c
index de9840f60100..63ca3b5be306 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -91,16 +91,6 @@ static struct hmm *hmm_register(struct mm_struct *mm)
         spin_lock_init(&hmm->lock);
         hmm->mm = mm;
 
-        /*
-         * We should only get here if hold the mmap_sem in write mode ie on
-         * registration of first mirror through hmm_mirror_register()
-         */
-        hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
-        if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
-                kfree(hmm);
-                return NULL;
-        }
-
         spin_lock(&mm->page_table_lock);
         if (!mm->hmm)
                 mm->hmm = hmm;
@@ -108,12 +98,27 @@ static struct hmm *hmm_register(struct mm_struct *mm)
                 cleanup = true;
         spin_unlock(&mm->page_table_lock);
 
-        if (cleanup) {
-                mmu_notifier_unregister(&hmm->mmu_notifier, mm);
-                kfree(hmm);
-        }
+        if (cleanup)
+                goto error;
+
+        /*
+         * We should only get here if hold the mmap_sem in write mode ie on
+         * registration of first mirror through hmm_mirror_register()
+         */
+        hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
+        if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
+                goto error_mm;
 
         return mm->hmm;
+
+error_mm:
+        spin_lock(&mm->page_table_lock);
+        if (mm->hmm == hmm)
+                mm->hmm = NULL;
+        spin_unlock(&mm->page_table_lock);
+error:
+        kfree(hmm);
+        return NULL;
 }
 
 void hmm_mm_destroy(struct mm_struct *mm)
@@ -278,12 +283,13 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
         if (!should_unregister || mm == NULL)
                 return;
 
+        mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
+
         spin_lock(&mm->page_table_lock);
         if (mm->hmm == hmm)
                 mm->hmm = NULL;
         spin_unlock(&mm->page_table_lock);
 
-        mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
         kfree(hmm);
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);