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

[Axel Rasmussen]

On Wed, Apr 28, 2021 at 4:09 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> 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>

Apologies, I should have added this line:

Cc: stable@xxxxxxxxxxxxxxx

I believe this fix ought to go into the 4.14 and later stable branches
(the commit referenced in Fixes: was introduced in 4.11).

I can resend with this included, if that would be easier.

> ---
>  mm/shmem.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..8def03d3f32a 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2375,8 +2375,18 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
>         pgoff_t offset, max_off;
>
>         ret = -ENOMEM;
> -       if (!shmem_inode_acct_block(inode, 1))
> +       if (!shmem_inode_acct_block(inode, 1)) {
> +               /*
> +                * We may have got a page, returned -ENOENT triggering a retry,
> +                * and now we find ourselves with -ENOMEM. Release the page, to
> +                * avoid a BUG_ON in our caller.
> +                */
> +               if (unlikely(*pagep)) {
> +                       put_page(*pagep);
> +                       *pagep = NULL;
> +               }
>                 goto out;
> +       }
>
>         if (!*pagep) {
>                 page = shmem_alloc_page(gfp, info, pgoff);
> --
> 2.31.1.498.g6c1eba8ee3d-goog
>