virtiofsd: Add an option to enable/disable security label

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>
Message-Id: <20220208204813.682906-11-vgoyal@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

3 files changed, 48 insertions, 0 deletions

diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst
index 07ac0be..0c05602 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 a8295d9..e226fc5 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 f5d584e..4742be1 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,
@@ -4288,6 +4302,7 @@ 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;