When serving TailFS shares, tailscaled executes another tailscaled to act as a
file server. It attempts to execute this child process as an unprivileged user
using sudo -u. This is important to avoid accessing files as root, which would
result in potential privilege escalation.
Previously, tailscaled assumed that it was running as someone who can sudo -u,
and would fail if it was unable to sudo -u.
With this commit, if tailscaled is unable to sudo -u as the requested user, and
tailscaled is not running as root, then tailscaled executes the the file server
process under the same identity that ran tailscaled, since this is already an
unprivileged identity.
In the unlikely event that tailscaled is running as root but is unable to
sudo -u, it will refuse to run the child file server process in order to avoid
privilege escalation.
Updates tailscale/corp#16827
Signed-off-by: Percy Wegmann <percy@tailscale.com>
Instead of modeling remote WebDAV servers as actual
webdav.FS instances, we now just proxy traffic to them.
This not only simplifies the code, but it also allows
WebDAV locking to work correctly by making sure locks are
handled by the servers that need to (i.e. the ones actually
serving the files).
Updates tailscale/corp#16827
Signed-off-by: Percy Wegmann <percy@tailscale.com>
