Re: [PATCH RESEND V12 7/8] fuse: Use daemon creds in passthrough mode

[Peng Tao]

On Mon, Jan 25, 2021 at 11:31 PM Alessio Balsini <balsini@xxxxxxxxxxx> wrote:
>
> When using FUSE passthrough, read/write operations are directly
> forwarded to the lower file system file through VFS, but there is no
> guarantee that the process that is triggering the request has the right
> permissions to access the lower file system. This would cause the
> read/write access to fail.
>
> In passthrough file systems, where the FUSE daemon is responsible for
> the enforcement of the lower file system access policies, often happens
> that the process dealing with the FUSE file system doesn't have access
> to the lower file system.
> Being the FUSE daemon in charge of implementing the FUSE file
> operations, that in the case of read/write operations usually simply
> results in the copy of memory buffers from/to the lower file system
> respectively, these operations are executed with the FUSE daemon
> privileges.
>
> This patch adds a reference to the FUSE daemon credentials, referenced
> at FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() time so that they can be used
> to temporarily raise the user credentials when accessing lower file
> system files in passthrough.
> The process accessing the FUSE file with passthrough enabled temporarily
> receives the privileges of the FUSE daemon while performing read/write
> operations. Similar behavior is implemented in overlayfs.
> These privileges will be reverted as soon as the IO operation completes.
> This feature does not provide any higher security privileges to those
> processes accessing the FUSE file system with passthrough enabled. This
> is because it is still the FUSE daemon responsible for enabling or not
> the passthrough feature at file open time, and should enable the feature
> only after appropriate access policy checks.
>
> Signed-off-by: Alessio Balsini <balsini@xxxxxxxxxxx>
> ---
>  fs/fuse/fuse_i.h      |  5 ++++-
>  fs/fuse/passthrough.c | 11 +++++++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index c4730d893324..815af1845b16 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -182,10 +182,13 @@ struct fuse_release_args;
>
>  /**
>   * Reference to lower filesystem file for read/write operations handled in
> - * passthrough mode
> + * passthrough mode.
> + * This struct also tracks the credentials to be used for handling read/write
> + * operations.
>   */
>  struct fuse_passthrough {
>         struct file *filp;
> +       struct cred *cred;
>  };
>
>  /** FUSE specific file data */
> diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> index c7fa1eeb7639..24866c5fe7e2 100644
> --- a/fs/fuse/passthrough.c
> +++ b/fs/fuse/passthrough.c
> @@ -52,6 +52,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
>                                    struct iov_iter *iter)
>  {
>         ssize_t ret;
> +       const struct cred *old_cred;
>         struct file *fuse_filp = iocb_fuse->ki_filp;
>         struct fuse_file *ff = fuse_filp->private_data;
>         struct file *passthrough_filp = ff->passthrough.filp;
> @@ -59,6 +60,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
>         if (!iov_iter_count(iter))
>                 return 0;
>
> +       old_cred = override_creds(ff->passthrough.cred);
>         if (is_sync_kiocb(iocb_fuse)) {
>                 ret = vfs_iter_read(passthrough_filp, iter, &iocb_fuse->ki_pos,
>                                     iocb_to_rw_flags(iocb_fuse->ki_flags,
> @@ -77,6 +79,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
>                 if (ret != -EIOCBQUEUED)
>                         fuse_aio_cleanup_handler(aio_req);
>         }
> +       revert_creds(old_cred);
cred should be reverted when kmalloc() fails above.

Cheers,
Tao
-- 
Into Sth. Rich & Strange