From: Ralph Campbell rcampbell@nvidia.com Subject: 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 ---

mm/hmm.c | 36 +++++++++++++++++++++--------------- 1 file changed, 21 insertions(+), 15 deletions(-)

--- a/mm/hmm.c~mm-hmm-fix-race-between-hmm_mirror_unregister-and-mmu_notifier-callback +++ a/mm/hmm.c @@ -91,16 +91,6 @@ static struct hmm *hmm_register(struct m 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 m 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_mi 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); _