From: Andrea Arcangeli aarcange@redhat.com Subject: userfaultfd: clear the vma->vm_userfaultfd_ctx if UFFD_EVENT_FORK fails

The previous fix 384632e67e0829 ("userfaultfd: non-cooperative: fix fork use after free") corrected the refcounting in case of UFFD_EVENT_FORK failure for the fork userfault paths. That still didn't clear the vma->vm_userfaultfd_ctx of the vmas that were set to point to the aborted new uffd ctx earlier in dup_userfaultfd.

Link: http://lkml.kernel.org/r/20171223002505.593-2-aarcange@redhat.com Signed-off-by: Andrea Arcangeli aarcange@redhat.com Reported-by: syzbot syzkaller@googlegroups.com Reviewed-by: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Eric Biggers ebiggers3@gmail.com Cc: Dmitry Vyukov dvyukov@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/userfaultfd.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-)

diff -puN fs/userfaultfd.c~userfaultfd-clear-the-vma-vm_userfaultfd_ctx-if-uffd_event_fork-fails fs/userfaultfd.c --- a/fs/userfaultfd.c~userfaultfd-clear-the-vma-vm_userfaultfd_ctx-if-uffd_event_fork-fails +++ a/fs/userfaultfd.c @@ -570,11 +570,14 @@ out: static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx, struct userfaultfd_wait_queue *ewq) { + struct userfaultfd_ctx *release_new_ctx; + if (WARN_ON_ONCE(current->flags & PF_EXITING)) goto out;

ewq->ctx = ctx; init_waitqueue_entry(&ewq->wq, current); + release_new_ctx = NULL;

spin_lock(&ctx->event_wqh.lock); /* @@ -601,8 +604,7 @@ static void userfaultfd_event_wait_compl new = (struct userfaultfd_ctx *) (unsigned long) ewq->msg.arg.reserved.reserved1; - - userfaultfd_ctx_put(new); + release_new_ctx = new; } break; } @@ -617,6 +619,20 @@ static void userfaultfd_event_wait_compl __set_current_state(TASK_RUNNING); spin_unlock(&ctx->event_wqh.lock);

+ if (release_new_ctx) { + struct vm_area_struct *vma; + struct mm_struct *mm = release_new_ctx->mm; + + /* the various vma->vm_userfaultfd_ctx still points to it */ + down_write(&mm->mmap_sem); + for (vma = mm->mmap; vma; vma = vma->vm_next) + if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) + vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; + up_write(&mm->mmap_sem); + + userfaultfd_ctx_put(release_new_ctx); + } + /* * ctx may go away after this if the userfault pseudo fd is * already released. _