Re: [PATCH] virtiofsd: Allow capability restriction to be turned off

[Vivek Goyal]

On Fri, Jun 19, 2020 at 06:18:09PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Commit a59feb483b8fae24d043 dropped most capabilities at startup,
> since in most cases virtiofsd doesn't need them.  Unfortunately we found
> a case that needs CAP_SYS_ADMIN - setting trusted extended attributes.
> It's rare to need it, and the clean fix for it is going to be more
> complicated, but as a way for existing users to continue, allow the
> capability restriction to be turned off by adding
> 
>   -o no_restrictcaps
> 
> We still drop the capabilities by default since this seems much safer;
> even if we don't have an explicit problem to fix.
> 

Hi David,

In one of the emails, Daniel suggested option "--trusted-xattrs=on|off". 
How about we use that and if that option is specified, we automatically
give CAP_SYS_ADMIN as well. Something to consider.

This probably could be extended later to "--trusted-xattrs=on|off|remapped"
when we have the capability to remap. And in case of "off/remap" we can take
away CAP_SYS_ADMIN. 

Vivek

> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  docs/tools/virtiofsd.rst         | 3 +++
>  tools/virtiofsd/helper.c         | 3 +++
>  tools/virtiofsd/passthrough_ll.c | 8 +++++++-
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst
> index 378594c422..8819d7d19e 100644
> --- a/docs/tools/virtiofsd.rst
> +++ b/docs/tools/virtiofsd.rst
> @@ -81,6 +81,9 @@ Options
>      Enable/disable extended attributes (xattr) on files and directories.  The
>      default is ``no_xattr``.
>  
> +  * restrictcaps|no_restrictcaps\n"
> +    Restrict capabilities at startup.  The default is ``restrictcaps``.
> +
>  .. option:: --socket-path=PATH
>  
>    Listen on vhost-user UNIX domain socket at PATH.
> diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
> index 00a1ef666a..51ed9fbed0 100644
> --- a/tools/virtiofsd/helper.c
> +++ b/tools/virtiofsd/helper.c
> @@ -174,6 +174,9 @@ void fuse_cmdline_help(void)
>             "                               default: no_writeback\n"
>             "    -o xattr|no_xattr          enable/disable xattr\n"
>             "                               default: no_xattr\n"
> +           "    -o restrictcaps|no_restrictcaps\n"
> +           "                               restrict capabilities at 
> startup\n"
> +           "                               default: restrictcaps\n"
>             "    --rlimit-nofile=<num>      set maximum number of file 
> descriptors\n"
>             "                               (0 leaves rlimit unchanged)\n"
>             "                               default: min(1000000, fs.file-max 
> - 16384)\n"
> diff --git a/tools/virtiofsd/passthrough_ll.c 
> b/tools/virtiofsd/passthrough_ll.c
> index 2ce7c96085..8dee657b19 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -144,6 +144,7 @@ struct lo_data {
>      int flock;
>      int posix_lock;
>      int xattr;
> +    int restrictcaps;
>      char *source;
>      double timeout;
>      int cache;
> @@ -170,6 +171,8 @@ static const struct fuse_opt lo_opts[] = {
>      { "no_posix_lock", offsetof(struct lo_data, posix_lock), 0 },
>      { "xattr", offsetof(struct lo_data, xattr), 1 },
>      { "no_xattr", offsetof(struct lo_data, xattr), 0 },
> +    { "restrictcaps", offsetof(struct lo_data, restrictcaps), 1 },
> +    { "no_restrictcaps", offsetof(struct lo_data, restrictcaps), 0 },
>      { "timeout=%lf", offsetof(struct lo_data, timeout), 0 },
>      { "timeout=", offsetof(struct lo_data, timeout_set), 1 },
>      { "cache=none", offsetof(struct lo_data, cache), CACHE_NONE },
> @@ -2615,7 +2618,9 @@ static void setup_sandbox(struct lo_data *lo, struct 
> fuse_session *se,
>      setup_namespaces(lo, se);
>      setup_mounts(lo->source);
>      setup_seccomp(enable_syslog);
> -    setup_capabilities();
> +    if (lo->restrictcaps) {
> +        setup_capabilities();
> +    }
>  }
>  
>  /* Set the maximum number of open file descriptors */
> @@ -2764,6 +2769,7 @@ int main(int argc, char *argv[])
>          .writeback = 0,
>          .posix_lock = 1,
>          .proc_self_fd = -1,
> +        .restrictcaps = 1,
>      };
>      struct lo_map_elem *root_elem;
>      int ret = -1;
> -- 
> 2.26.2
>