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

[Kefeng Wang]

On 2019/8/21 0:02, Oleg Nesterov wrote:
> 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 
> 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 
> ---
>  fs/userfaultfd.c | 25 +
>  1 file changed, 13 insertions(+), 12 deletions(-)

Tested on lts4.4 and 5.3-rc4, Thanks.

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

[Andrea Arcangeli]

On Tue, Aug 20, 2019 at 06:02:38PM +0200, Oleg Nesterov wrote:
> 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 
> 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 
> ---
>  fs/userfaultfd.c | 25 +
>  1 file changed, 13 insertions(+), 12 deletions(-)

Reviewed-by: Andrea Arcangeli 

Thanks,
Andrea

[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 
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 
---
 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(>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(>mmap_sem);
mmput(mm);
 wakeup:
-- 
2.5.0