Re: [PATCH] virtiofsd: prevent opening of special files (CVE-2020-35517)

[Daniel P . Berrangé]

On Thu, Jan 21, 2021 at 02:44:29PM +0000, Stefan Hajnoczi wrote:
> A well-behaved FUSE client does not attempt to open special files with
> FUSE_OPEN because they are handled on the client side (e.g. device nodes
> are handled by client-side device drivers).
> 
> The check to prevent virtiofsd from opening special files is missing in
> a few cases, most notably FUSE_OPEN. A malicious client can cause
> virtiofsd to open a device node, potentially allowing the guest to
> escape. This can be exploited by a modified guest device driver. It is
> not exploitable from guest userspace since the guest kernel will handle
> special files inside the guest instead of sending FUSE requests.
> 
> This patch adds the missing checks to virtiofsd. This is a short-term
> solution because it does not prevent a compromised virtiofsd process
> from opening device nodes on the host.
> 
> Reported-by: Alex Xu <alex@alxu.ca>
> Fixes: CVE-2020-35517
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> This issue was diagnosed on public IRC and is therefore already known
> and not embargoed.
> 
> A stronger fix, and the long-term solution, is for users to mount the
> shared directory and any sub-mounts with nodev, as well as nosuid and
> noexec. Unfortunately virtiofsd cannot do this automatically because
> bind mounts added by the user after virtiofsd has launched would not be
> detected. I suggest the following:
> 
> 1. Modify libvirt and Kata Containers to explicitly set these mount
>    options.
> 2. Then modify virtiofsd to check that the shared directory has the
>    necessary options at startup. Refuse to start if the options are
>    missing so that the user is aware of the security requirements.
> 
> As a bonus this also increases the likelihood that other host processes
> besides virtiofsd will be protected by nosuid/noexec/nodev so that a
> malicious guest cannot drop these files in place and then arrange for a
> host process to come across them.
> 
> Additionally, user namespaces have been discussed. They seem like a
> worthwhile addition as an unprivileged or privilege-separated mode
> although there are limitations with respect to security xattrs and the
> actual uid/gid stored on the host file system not corresponding to the
> guest uid/gid.
> ---
>  tools/virtiofsd/passthrough_ll.c | 84 +++++++++++++++++++++-----------
>  1 file changed, 56 insertions(+), 28 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c 
> b/tools/virtiofsd/passthrough_ll.c
> index 5fb36d9407..e08352d649 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -555,6 +555,29 @@ static int lo_fd(fuse_req_t req, fuse_ino_t ino)
>      return fd;
>  }
>  
> +/*
> + * Open a file descriptor for an inode. Returns -EBADF if the inode is not a
> + * regular file or a directory. Use this helper function instead of raw
> + * openat(2) to prevent security issues when a malicious client opens special
> + * files such as block device nodes.
> + */
> +static int lo_inode_open(struct lo_data *lo, struct lo_inode *inode,
> +                         int open_flags)
> +{
> +    g_autofree char *fd_str = g_strdup_printf("%d", inode->fd);
> +    int fd;
> +
> +    if (!S_ISREG(inode->filetype) && !S_ISDIR(inode->filetype)) {
> +        return -EBADF;
> +    }
> +
> +    fd = openat(lo->proc_self_fd, fd_str, open_flags);

Whats the intended behaviour with symlinks ?  Do we need to
allow S_ISLNK, or are we assuming the symlink has already
been expanded to the target file by this point ? If the latter
adding a comment about this would be useful.

> +    if (fd < 0) {
> +        return -errno;
> +    }
> +    return fd;
> +}
> +

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|