[PATCH v2 1/3] udmabuf: fix racy memfd sealing check

[Joel Fernandes]

On Wed, Dec 4, 2024 at 11:27 AM Jann Horn <jannh at google.com> wrote:
>
> The current check_memfd_seals() is racy: Since we first do
> check_memfd_seals() and then udmabuf_pin_folios() without holding any
> relevant lock across both, F_SEAL_WRITE can be set in between.
> This is problematic because we can end up holding pins to pages in a
> write-sealed memfd.
>
> Fix it using the inode lock, that's probably the easiest way.
> In the future, we might want to consider moving this logic into memfd,
> especially if anyone else wants to use memfd_pin_folios().

I am curious, why is it not possible to have a reproducer for this
issue, is it not reproducible and is theoretical?

thanks,

 - Joel

>
> Reported-by: Julian Orth <ju.orth at gmail.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106
> Closes: https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTgpD+StkV9w@mail.gmail.com
> Fixes: fbb0de795078 ("Add udmabuf misc device")
> Cc: stable at vger.kernel.org
> Signed-off-by: Jann Horn <jannh at google.com>
> ---
>  drivers/dma-buf/udmabuf.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 8ce1f074c2d32a0a9f59ff7184359e37d56548c6..c1d8c2766d6d36fc5fe1b3d73057f6e01ec6678f 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -436,14 +436,19 @@ static long udmabuf_create(struct miscdevice *device,
>                         goto err;
>                 }
>
> +               /*
> +                * Take the inode lock to protect against concurrent
> +                * memfd_add_seals(), which takes this lock in write mode.
> +                */
> +               inode_lock_shared(file_inode(memfd));
>                 ret = check_memfd_seals(memfd);
> -               if (ret < 0) {
> -                       fput(memfd);
> -                       goto err;
> -               }
> +               if (ret)
> +                       goto out_unlock;
>
>                 ret = udmabuf_pin_folios(ubuf, memfd, list[i].offset,
>                                          list[i].size, folios);
> +out_unlock:
> +               inode_unlock_shared(file_inode(memfd));
>                 fput(memfd);
>                 if (ret)
>                         goto err;
>
> --
> 2.47.0.338.g60cca15819-goog
>