Run gateway sessions as the configured sandbox user

src/providers/docker_sbx/process.py

@@ -3,22 +3,33 @@
 import fcntl
 import os
 import pty
+import shlex
 import struct
 import termios
 
 from providers.base import SandboxProcess, TerminalSize
 from providers.exceptions import ProviderTransportError
 
+from .settings import DockerSbxSettings
+
 
 def _set_terminal_size(descriptor: int, size: TerminalSize) -> None:
     winsize = struct.pack("HHHH", size.rows, size.columns, 0, 0)
     fcntl.ioctl(descriptor, termios.TIOCSWINSZ, winsize)
 
 
-def _session_script(name: str, command: str | None) -> str:
+def _session_script(name: str, command: str | None, user: str) -> str:
+    # The exec enters as root and prepares the per-host home. For a non-root
+    # user it then drops with `su -m`, which keeps the exported HOME and,
+    # through PAM, still applies /etc/environment.
     home = f"/home/{name}"
     payload = command if command is not None else "exec bash -l"
-    return f"mkdir -p {home} && cd {home} && export HOME={home}\n{payload}"
+    prepare = f"mkdir -p {home} && cd {home} && export HOME={home}"
+    if user == "root":
+        return f"{prepare}\n{payload}"
+    owner = shlex.quote(user)
+    drop = f"exec su -m {owner} -s /bin/bash -c {shlex.quote(payload)}"
+    return f"{prepare} && chown {owner} {home}\n{drop}"
 
 
 class SbxExecProcess(SandboxProcess):
@@ -48,10 +59,11 @@ async def open(
         command: str | None,
         terminal: TerminalSize | None,
     ) -> "SbxExecProcess":
+        user = DockerSbxSettings().ssh_username
         argv = ["sbx", "exec", "--interactive"]
         if terminal:
             argv.append("--tty")
-        argv.extend([name, "bash", "-l", "-c", _session_script(name, command)])
+        argv.extend([name, "bash", "-l", "-c", _session_script(name, command, user)])
         environment = {**os.environ, "SBX_NO_TELEMETRY": "1"}
 
         try:

src/providers/docker_sbx/tests/test_process.py

@@ -1,5 +1,6 @@
 import asyncio
 import os
+import shlex
 
 import pytest
 
@@ -48,7 +49,7 @@ async def test_exec_session_bridges_pipes_in_both_directions(monkeypatch):
         "-l",
         "-c",
     )
-    assert captured["argv"][7] == _session_script("sb-test", "true")
+    assert captured["argv"][7] == _session_script("sb-test", "true", "root")
     assert output == b"round-trip"
     assert status == 5
 
@@ -85,7 +86,7 @@ async def test_terminal_request_allocates_a_real_pty_at_the_requested_size(monke
 
     assert "--tty" in captured["argv"]
     assert captured["argv"][-4:-1] == ("bash", "-l", "-c")
-    assert captured["argv"][-1] == _session_script("sb-test", None)
+    assert captured["argv"][-1] == _session_script("sb-test", None, "root")
     assert b"ISTTY" in output
     assert b"42 101" in output
     assert status == 3
@@ -109,7 +110,7 @@ async def test_aclose_releases_the_terminal_descriptor(monkeypatch):
 
 
 def test_session_prepares_the_per_host_home_before_the_command():
-    script = _session_script("sb-abc", "git status")
+    script = _session_script("sb-abc", "git status", "root")
     assert "mkdir -p /home/sb-abc" in script
     assert "export HOME=/home/sb-abc" in script
     # The home exists and HOME is set before the command runs.
@@ -118,11 +119,58 @@ def test_session_prepares_the_per_host_home_before_the_command():
 
 
 def test_session_without_a_command_runs_a_login_shell_in_the_home():
-    script = _session_script("sb-abc", None)
+    script = _session_script("sb-abc", None, "root")
     assert "mkdir -p /home/sb-abc" in script
     assert script.endswith("exec bash -l")
 
 
+def test_root_session_is_byte_identical_to_the_plain_home_setup():
+    # The default install must not change: no chown, no su.
+    assert _session_script("sb-abc", "git status", "root") == (
+        "mkdir -p /home/sb-abc && cd /home/sb-abc && export HOME=/home/sb-abc\ngit status"
+    )
+    assert "su" not in _session_script("sb-abc", None, "root")
+
+
+def test_non_root_session_chowns_the_home_and_drops_to_the_user():
+    script = _session_script("sb-abc", "agent run --flag", "druks")
+    # Root prepares and gives the user the home before it drops.
+    assert "chown druks /home/sb-abc" in script
+    assert script.index("mkdir -p /home/sb-abc") < script.index("su -m druks")
+    assert script.index("chown druks") < script.index("su -m druks")
+    # su -m keeps the exported HOME; the payload is quoted for /bin/bash -c.
+    assert f"exec su -m druks -s /bin/bash -c {shlex.quote('agent run --flag')}" in script
+
+
+def test_non_root_interactive_session_drops_to_a_login_shell():
+    script = _session_script("sb-abc", None, "druks")
+    assert f"exec su -m druks -s /bin/bash -c {shlex.quote('exec bash -l')}" in script
+
+
+def test_non_root_sftp_backing_shell_runs_as_the_user():
+    # The SFTP backend passes its server command through the same open();
+    # thus its backing shell drops to the user too.
+    script = _session_script("sb-abc", "exec /usr/lib/openssh/sftp-server", "druks")
+    assert "exec su -m druks -s /bin/bash -c" in script
+    assert shlex.quote("exec /usr/lib/openssh/sftp-server") in script
+
+
+async def test_open_runs_the_session_as_the_configured_user(monkeypatch):
+    monkeypatch.setenv("DOCKER_SBX_SSH_USERNAME", "druks")
+    captured: dict = {}
+    monkeypatch.setattr(
+        "providers.docker_sbx.process.asyncio.create_subprocess_exec",
+        _stub_sbx_with("exit 0", captured),
+    )
+
+    session = await SbxExecProcess.open("sb-test", command="id -un", terminal=None)
+    await session.wait()
+    await session.aclose()
+
+    assert captured["argv"][7] == _session_script("sb-test", "id -un", "druks")
+    assert "exec su -m druks" in captured["argv"][7]
+
+
 async def test_missing_sbx_binary_maps_to_transport_error(monkeypatch):
     async def fail_exec(*argv, **kwargs):
         raise FileNotFoundError("sbx")