Re: [PATCH v2] userfaultfd: release page in error path to avoid BUG_ON

[Peter Xu]

On Wed, Apr 28, 2021 at 04:08:58PM -0700, Axel Rasmussen wrote:
> Consider the following sequence of events:
> 
> 1. Userspace issues a UFFD ioctl, which ends up calling into
>    shmem_mfill_atomic_pte(). We successfully account the blocks, we
>    shmem_alloc_page(), but then the copy_from_user() fails. We return
>    -ENOENT. We don't release the page we allocated.
> 2. Our caller detects this error code, tries the copy_from_user() after
>    dropping the mmap_lock, and retries, calling back into
>    shmem_mfill_atomic_pte().
> 3. Meanwhile, let's say another process filled up the tmpfs being used.
> 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and
>    immediately returns - without releasing the page.
> 
> This triggers a BUG_ON in our caller, which asserts that the page
> should always be consumed, unless -ENOENT is returned.
> 
> To fix this, detect if we have such a "dangling" page when accounting
> fails, and if so, release it before returning.
> 
> Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

Thanks,

-- 
Peter Xu