[PATCH] userfaultfd_release: always remove uffd flags and clear vm_userfaultfd_ctx

[Oleg Nesterov]

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.

Reported-by: Kefeng Wang <wangkefeng.w...@huawei.com>
Fixes: 04f5866e41fb ("coredump: fix race condition between 
mmget_not_zero()/get_task_mm() and core dumping")
Cc: sta...@vger.kernel.org
Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
 fs/userfaultfd.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ccbdbd6..fe6d804 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:
-- 
2.5.0