Clean up wayland device jail

The jail for the wayland device used chown to ensure that its jail had
the proper permissions for the wayland socket to be bind mounted into
it.  This creates some unnecessary complexity because it requires
careful management of the user and group and crosvm runs as (a non-root
user cannot change the owner of a directory) or that crosvm has the
CAP_CHOWN capability.

Instead of trying to make the permissions fit, just have the jail mount
a small tmpfs over the jail's root directory.  This is one of the things
that a process inside a user namespace has the ability to do.  Bind
mounting the wayland socket into this tmpfs then just works without any
other issues.

BUG=chromium:799523
TEST=linux vm boots with no errors

Change-Id: Ic2240f430c7fd332a15b4fcd4e52374799eb6c9d
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/851413
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>

1 files changed, 8 insertions, 16 deletions

diff --git a/src/linux.rs b/src/linux.rs
index fbd8e1c..46faed2 100644
--- a/src/linux.rs
+++ b/src/linux.rs
@@ -360,7 +360,6 @@ fn setup_mmio_bus(cfg: &Config,
         }
     }
 
-    let wl_root = TempDir::new(&PathBuf::from("/tmp/wl_root")).map_err(Error::WaylandTempDir)?;
     if let Some(wayland_socket_path) = cfg.wayland_socket_path.as_ref() {
         let jailed_wayland_path = Path::new("/wayland-0");
 
@@ -375,9 +374,15 @@ fn setup_mmio_bus(cfg: &Config,
                                       .map_err(Error::WaylandDeviceNew)?);
 
         let jail = if cfg.multiprocess {
-            let wl_root_path = wl_root.as_path().unwrap(); // Won't fail if new succeeded.
             let policy_path: PathBuf = cfg.seccomp_policy_dir.join("wl_device.policy");
-            let mut jail = create_base_minijail(wl_root_path, &policy_path)?;
+            let mut jail = create_base_minijail(empty_root_path, &policy_path)?;
+
+            // Create a tmpfs in the device's root directory so that we can bind mount the
+            // wayland socket into it.  The size=67108864 is size=64*1024*1024 or size=64MB.
+            jail.mount_with_data(Path::new("none"), Path::new("/"), "tmpfs",
+                                 (libc::MS_NOSUID | libc::MS_NODEV | libc::MS_NOEXEC) as usize,
+                                 "size=67108864")
+                .unwrap();
 
             // Bind mount the wayland socket into jail's root. This is necessary since each
             // new wayland context must open() the socket.
@@ -399,13 +404,6 @@ fn setup_mmio_bus(cfg: &Config,
                     geteuid()
                 }
             };
-            let crosvm_gid = match get_group_id(&crosvm_user_group) {
-                Ok(u) => u,
-                Err(e) => {
-                    warn!("falling back to current group id for Wayland: {:?}", e);
-                    getegid()
-                }
-            };
             jail.change_uid(crosvm_uid);
             jail.change_gid(wayland_gid);
             jail.uidmap(&format!("{0} {0} 1", crosvm_uid))
@@ -413,12 +411,6 @@ fn setup_mmio_bus(cfg: &Config,
             jail.gidmap(&format!("{0} {0} 1", wayland_gid))
                 .map_err(Error::SettingGidMap)?;
 
-            // chown the root directory for the jail so we can actually bind mount the socket.
-            let wayland_root_cstr = CString::new(wl_root_path.as_os_str().to_str().unwrap())
-                .unwrap();
-            chown(&wayland_root_cstr, crosvm_uid, crosvm_gid)
-                .map_err(Error::ChownWaylandRoot)?;
-
             Some(jail)
         } else {
             None