Re: [PATCH v2 2/2] fuse: Support FS_IOC_GET_ENCRYPTION_POLICY_EX

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, Dec 8, 2020 at 10:38 AM Chirantan Ekbote <chirantan@chromium.org> wrote:
>
> Chrome OS would like to support this ioctl when passed through the fuse
> driver. However since it is dynamically sized, we can't rely on the
> length encoded in the command.  Instead check the `policy_size` field of
> the user provided parameter to get the max length of the data returned
> by the server.

I'd also maximize the length at sizeof(union fscrypt_policy).  I.e.
virtiofs doesn't need to support higher level versions than the client
kernel supports.

Also, I'm thinking about whether it's safe to enable in plain fuse in
addition to virtiofs.  I don't see a reason for not doing so, but
maybe it makes sense to keep disabled until a use case comes up.

Thanks,
Miklos


>
> Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
> ---
>  fs/fuse/file.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 69cffb77a0b25..b64ff7f2fe4dd 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -19,6 +19,7 @@
>  #include <linux/falloc.h>
>  #include <linux/uio.h>
>  #include <linux/fs.h>
> +#include <linux/fscrypt.h>
>
>  static struct page **fuse_pages_alloc(unsigned int npages, gfp_t flags,
>                                       struct fuse_page_desc **desc)
> @@ -2710,6 +2711,21 @@ static int fuse_get_ioctl_len(unsigned int cmd, unsigned long arg, size_t *len)
>         case FS_IOC_SETFLAGS:
>                 *len = sizeof(int);
>                 break;
> +       case FS_IOC_GET_ENCRYPTION_POLICY_EX: {
> +               __u64 policy_size;
> +               struct fscrypt_get_policy_ex_arg __user *uarg =
> +                       (struct fscrypt_get_policy_ex_arg __user *)arg;
> +
> +               if (copy_from_user(&policy_size, &uarg->policy_size,
> +                                  sizeof(policy_size)))
> +                       return -EFAULT;
> +
> +               if (policy_size > SIZE_MAX - sizeof(policy_size))
> +                       return -EINVAL;
> +
> +               *len = sizeof(policy_size) + policy_size;
> +               break;
> +       }
>         default:
>                 *len = _IOC_SIZE(cmd);
>                 break;
> --
> 2.29.2.576.ga3fc446d84-goog
>