userfaultfd_release: always remove uffd flags and clear vm_userfaultfd_ctx

userfaultfd_release() should clear vm_flags/vm_userfaultfd_ctx even if
mm->core_state != NULL.

Otherwise a page fault can see userfaultfd_missing() == T and use an
already freed userfaultfd_ctx.

Link: http://lkml.kernel.org/r/20190820160237.GB4983@redhat.com
Fixes: 04f5866e41fb ("coredump: fix race condition between mmget_not_zero()/get_task_mm() and core dumping")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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, 13 insertions, 12 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ccbdbd62f0d8..fe6d804a38dc 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -880,6 +880,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
         /* len == 0 means wake all */
         struct userfaultfd_wake_range range = { .len = 0, };
         unsigned long new_flags;
+        bool still_valid;
 
         WRITE_ONCE(ctx->released, true);
 
@@ -895,8 +896,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
          * taking the mmap_sem for writing.
          */
         down_write(&mm->mmap_sem);
-        if (!mmget_still_valid(mm))
-                goto skip_mm;
+        still_valid = mmget_still_valid(mm);
         prev = NULL;
         for (vma = mm->mmap; vma; vma = vma->vm_next) {
                 cond_resched();
@@ -907,19 +907,20 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
                         continue;
                 }
                 new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
-                prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
-                                 new_flags, vma->anon_vma,
-                                 vma->vm_file, vma->vm_pgoff,
-                                 vma_policy(vma),
-                                 NULL_VM_UFFD_CTX);
-                if (prev)
-                        vma = prev;
-                else
-                        prev = vma;
+                if (still_valid) {
+                        prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
+                                         new_flags, vma->anon_vma,
+                                         vma->vm_file, vma->vm_pgoff,
+                                         vma_policy(vma),
+                                         NULL_VM_UFFD_CTX);
+                        if (prev)
+                                vma = prev;
+                        else
+                                prev = vma;
+                }
                 vma->vm_flags = new_flags;
                 vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
         }
-skip_mm:
         up_write(&mm->mmap_sem);
         mmput(mm);
 wakeup: