Re: [PATCH v6 10/10] virtiofsd: Add an option to enable/disable security label

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Feb 14, 2022 at 01:32:38PM +0000, Dr. David Alan Gilbert wrote:
> * Vivek Goyal (vgoyal@redhat.com) wrote:
> > Provide an option "-o security_label/no_security_label" to enable/disable
> > security label functionality. By default these are turned off.
> > 
> > If enabled, server will indicate to client that it is capable of handling
> > one security label during file creation. Typically this is expected to
> > be a SELinux label. File server will set this label on the file. It will
> > try to set it atomically wherever possible. But its not possible in
> > all the cases.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> OK, but you have missed some of the docs typos I mentined in the last
> review; they can be cleared up any time.

Hi David,

I could not find any comments in V5 w.r.t doc typos. I am not sure which
email I have missed.

Anyway, will be nice if I can take care of these typos in a follow up
patch and these patches can be merged.

Thanks
Vivek
> 
> > ---
> >  docs/tools/virtiofsd.rst         | 32 ++++++++++++++++++++++++++++++++
> >  tools/virtiofsd/helper.c         |  1 +
> >  tools/virtiofsd/passthrough_ll.c | 15 +++++++++++++++
> >  3 files changed, 48 insertions(+)
> > 
> > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst
> > index 07ac0be551..0c0560203c 100644
> > --- a/docs/tools/virtiofsd.rst
> > +++ b/docs/tools/virtiofsd.rst
> > @@ -104,6 +104,13 @@ Options
> >    * posix_acl|no_posix_acl -
> >      Enable/disable posix acl support.  Posix ACLs are disabled by default.
> >  
> > +  * security_label|no_security_label -
> > +    Enable/disable security label support. Security labels are disabled by
> > +    default. This will allow client to send a MAC label of file during
> > +    file creation. Typically this is expected to be SELinux security
> > +    label. Server will try to set that label on newly created file
> > +    atomically wherever possible.
> > +
> >  .. option:: --socket-path=PATH
> >  
> >    Listen on vhost-user UNIX domain socket at PATH.
> > @@ -348,6 +355,31 @@ client arguments or lists returned from the host.  This stops
> >  the client seeing any 'security.' attributes on the server and
> >  stops it setting any.
> >  
> > +SELinux support
> > +---------------
> > +One can enable support for SELinux by running virtiofsd with option
> > +"-o security_label". But this will try to save guest's security context
> > +in xattr security.selinux on host and it might fail if host's SELinux
> > +policy does not permit virtiofsd to do this operation.
> > +
> > +Hence, it is preferred to remap guest's "security.selinux" xattr to say
> > +"trusted.virtiofs.security.selinux" on host.
> > +
> > +"-o xattrmap=:map:security.selinux:trusted.virtiofs.:"
> > +
> > +This will make sure that guest and host's SELinux xattrs on same file
> > +remain separate and not interfere with each other. And will allow both
> > +host and guest to implement their own separate SELinux policies.
> > +
> > +Setting trusted xattr on host requires CAP_SYS_ADMIN. So one will need
> > +add this capability to daemon.
> > +
> > +"-o modcaps=+sys_admin"
> > +
> > +Giving CAP_SYS_ADMIN increases the risk on system. Now virtiofsd is more
> > +powerful and if gets compromised, it can do lot of damage to host system.
> > +So keep this trade-off in my mind while making a decision.
> > +
> >  Examples
> >  --------
> >  
> > diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
> > index a8295d975a..e226fc590f 100644
> > --- a/tools/virtiofsd/helper.c
> > +++ b/tools/virtiofsd/helper.c
> > @@ -187,6 +187,7 @@ void fuse_cmdline_help(void)
> >             "                               default: no_allow_direct_io\n"
> >             "    -o announce_submounts      Announce sub-mount points to the guest\n"
> >             "    -o posix_acl/no_posix_acl  Enable/Disable posix_acl. (default: disabled)\n"
> > +           "    -o security_label/no_security_label  Enable/Disable security label. (default: disabled)\n"
> >             );
> >  }
> >  
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index d49128a58d..f3ec6aafe5 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -181,6 +181,7 @@ struct lo_data {
> >      int user_posix_acl, posix_acl;
> >      /* Keeps track if /proc/<pid>/attr/fscreate should be used or not */
> >      bool use_fscreate;
> > +    int user_security_label;
> >  };
> >  
> >  static const struct fuse_opt lo_opts[] = {
> > @@ -215,6 +216,8 @@ static const struct fuse_opt lo_opts[] = {
> >      { "no_killpriv_v2", offsetof(struct lo_data, user_killpriv_v2), 0 },
> >      { "posix_acl", offsetof(struct lo_data, user_posix_acl), 1 },
> >      { "no_posix_acl", offsetof(struct lo_data, user_posix_acl), 0 },
> > +    { "security_label", offsetof(struct lo_data, user_security_label), 1 },
> > +    { "no_security_label", offsetof(struct lo_data, user_security_label), 0 },
> >      FUSE_OPT_END
> >  };
> >  static bool use_syslog = false;
> > @@ -808,6 +811,17 @@ static void lo_init(void *userdata, struct fuse_conn_info *conn)
> >          fuse_log(FUSE_LOG_DEBUG, "lo_init: disabling posix_acl\n");
> >          conn->want &= ~FUSE_CAP_POSIX_ACL;
> >      }
> > +
> > +    if (lo->user_security_label == 1) {
> > +        if (!(conn->capable & FUSE_CAP_SECURITY_CTX)) {
> > +            fuse_log(FUSE_LOG_ERR, "lo_init: Can not enable security label."
> > +                     " kernel does not support FUSE_SECURITY_CTX capability.\n");
> > +        }
> > +        conn->want |= FUSE_CAP_SECURITY_CTX;
> > +    } else {
> > +        fuse_log(FUSE_LOG_DEBUG, "lo_init: disabling security label\n");
> > +        conn->want &= ~FUSE_CAP_SECURITY_CTX;
> > +    }
> >  }
> >  
> >  static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
> > @@ -4279,6 +4293,7 @@ int main(int argc, char *argv[])
> >          .proc_self_task = -1,
> >          .user_killpriv_v2 = -1,
> >          .user_posix_acl = -1,
> > +        .user_security_label = -1,
> >      };
> >      struct lo_map_elem *root_elem;
> >      struct lo_map_elem *reserve_elem;
> > -- 
> > 2.34.1
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>